Variable scale and reference

[valentinsulzer]

Description

Adds scale and reference attributes to a variable, which are used to multiply and/or add a value to a variable to make the resulting numerics better conditioned.
Progress towards #2418

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Base: 99.72% // Head: 99.72% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (a880bd1) compared to base (c2c0f28).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           more-simplifications    #2440   +/-   ##
=====================================================
  Coverage                 99.72%   99.72%           
=====================================================
  Files                       258      258           
  Lines                     19211    19258   +47     
=====================================================
+ Hits                      19158    19205   +47     
  Misses                       53       53           

Impacted Files	Coverage Δ
...models/full_battery_models/lead_acid/basic_full.py	100.00% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_dfn.py	100.00% <ø> (ø)
..._battery_models/lithium_ion/basic_dfn_half_cell.py	99.01% <ø> (ø)
...lyte_conductivity/base_electrolyte_conductivity.py	100.00% <ø> (ø)
...dels/electrolyte_conductivity/full_conductivity.py	100.00% <ø> (ø)
pybamm/discretisations/discretisation.py	99.80% <100.00%> (+<0.01%)	⬆️
pybamm/expression_tree/concatenations.py	100.00% <100.00%> (ø)
pybamm/expression_tree/symbol.py	100.00% <100.00%> (ø)
pybamm/expression_tree/variable.py	100.00% <100.00%> (ø)
...e_potential_form/full_surface_form_conductivity.py	100.00% <100.00%> (ø)
... and 3 more

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[brosaplanella]

Looks good, but the dimensional DFN is not there yet, is it?

[brosaplanella]

Suggested change

	Whether the equations are initial conditions. If True, the equations are
	Whether the equations have initial conditions. If True, the equations are

[valentinsulzer]

This is meant to be are, this is the function that discretises the equations (rhs, algebraic, initial conditions) and there is a special case for initial conditions (reference only gets taken away from initial conditions)

[brosaplanella]

What's the meaning of Fc?

[valentinsulzer]

Faraday's constant times c

[valentinsulzer]

I removed the dimensional DFN since when we eventually dimensionalize all the models it will not be there, so I don't want people adding it to their code. This PR now only adds the scaling and reference