Squashed commit containing most changes made to components

Old commit message:
Added linearity attributes (and cleared up two typos).
I am uncertain about the correct approach to the expression component
(currently set any parameter’s is_linear to False).

I am also unsure about which parameters are linear for the heaviside,
voigt, RC, PESCORELineShape (with shirley) and EELS_vignetting.

"Better safe than sorry" attitude with linear components, but set heaviside linear.

Fixed linearity of expression

Temp fix for Expression, until removing is_linear = False attribute.

I feel like an idiot for writing what I did.

Parameters default to parameter.is_linear = False

Removed parameter.is_linear = False in favour of default setting = False.

Removed existance of .function method which existed for some reason in expression.

Removed linear=False from heaviside.

Fixed a typo on the Expression example

Sympy now checks Expression for linear components

Added tests

Simplified sympy check and moved it out of class

Better function name

Now correct check of linearity

Clarified code

Added a test case

Sympy extract constant part of expression with respect to variables

Made is_linear protected

Added constant term property to expression

Added docstring

Added check for only one linear parameter per component. + TIdy

Fixed tests and "one linear" check

Docstring + tidying

Fixed missing _is_linear

Offset is y=k, not y=k+x

set Offset _is_linear True

fixed (my) bug in Offset component

Fixed test failing

Added polynomial constant, needs a look

Corrected expression constant extractor + tests

indent typo

Working with constant term

Remove component._constant_parameters as it doesn't make sense if constant is free

Added three linear fitters

Added imports and docstring

Added tests

Some PEP changes

Added bounds to lsq_linear

Added test for bounded lsq_linear

Ensure that linear components aren't initialised at zero

Some formatting

Ensure components don't get added twice

Renamed "linear_lsq" to "linear"

Reordering priority of fitters

If fitter is None, use linear if model is linear

Supports case of one component being inactive

Fix fitting with inactive components for other fitters

one file changed?

check these changes

Fixed for SI and changed warnings to logging INFO

Removed print statement

Cleared up some code, fixed a bug

Cleaned code, fixed axis bug

Fixed linear fitting with bounds

made methods private

deleted obsolete method

fixed tests

some language and cleanup

Corrected binned signal fitting for linear

fixed truedivide error

Removed the other two forms of linear fitting. Can add later.

minor cleaning up

removed print statement

Added preliminary support for convolved components

Fixed transposing errors

Removed old function and print statements

fixed test error

Fixed changed signal range breaking

Removed remnants of other linear fitters

More remnants

autopep8

typo

Working for convolved, nonconvolved, free and fixed components

Add support for linear fitting with a free offset, by either component or parameter

Update p0 and subtract free offsets from model, adding it as a type of component instead

some tests

code reduced, but slow?

twinned case must be considered

Documentation 2.0. Thanks @tjof2

clearer docs

corrections to docs

Deleted some unnecessary lines from linear fitting docs

MODEL: Underscore bug

MODEL: default fitter no longer from prefs

MODEL: Working with twins but not in pseudocomponents

fix twin situation

fix twin situation

moved test files

removed old case for appending components

Change test case to use artificial data

add standard error

Remove old code

TEST: Add and remove test cases, and fix hydrogenic

Cleaned up linear fitting procedure, simplified linear model estimation procedure and formatted code

Removed unused code, cleaned up with pep8

moved error message

Rewrote component function calls to support multidimensional models

fixed small bugs with previous commits

more fixes to the previous, now working

made expression fix_free separation 2D-able

corrected shape

Add 2D model support for linear fitting

Added test cases for model2D linear fitting

model.py typo

hotfix for Zanetta

large multifit update - mostly working

need to fix expression multi

getting closer

 minor

new twin tactic

much changes

beginning of EELS CL support

further changes to eels

modified convolve. Nearly there

further GOS changes

hello

fixed Model2D

updated bleasdale to take parameters in function call

remove print

readied most comps for linear multifit

minor

fixing components and adding tests

remove unused code

docstring and PEP improvements

update docs

remove old lsq_linear reference

remove old import

fixed fit_component and signal_range