Feature/adding errors

[aoeftiger]

The PR adds an approach to add MAD-X lattice errors (from a cpymad exposed error table e.g. via ESAVE) to an existing line. For now this supports
a) XYShift for offsets (dx, dy in MAD-X),
b) SRotation for tilts (dpsi in MAD-X) and
c) multipole errors (k1-20l and k1-20sl in MAD-X).

The following should be confirmed before merging:

1. elements with both offset and tilt error: the inner tilt wrapping the element is wrapped by an outer offset: e.g. Multipole --> XYShift + SRotation + Multipole + SRotation + XYShift
2. the offset is inverted: a positive dx or dy error for a MAD-X element leads to a negative shift in the particle coordinate when entering
3. the tilt is inverted: a positive dpsi error for a MAD-X element leads to a negative angle in the SRotation when entering

[pep8speaks]

Hello @aoeftiger! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file pysixtrack/line.py:

Line 397:28: E127 continuation line over-indented for visual indent
Line 401:16: E713 test for membership should be 'not in'

Comment last updated at 2019-10-21 12:42:56 UTC

[aoeftiger]

sweet, I've adapted to PEP8 now

Hello @aoeftiger! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

* In the file [`pysixtrack/line.py`](https://github.com/rdemaria/pysixtrack/blob/2faab735cf7e0273a24f83de7412d5c9dccd326b/pysixtrack/line.py):

Line 312:5: E266 too many leading '#' for block comment
Line 390:26: E211 whitespace before '('
Line 395:16: E713 test for membership should be 'not in'
Line 419:40: E202 whitespace before ']'
Line 419:80: E501 line too long (88 > 79 characters)
Line 420:80: E501 line too long (88 > 79 characters)

[aoeftiger]

To be improved before merging:

1. integrate error handling for PySixTrack already into loader_madx functionality -- by reading out errors via new cpymad functionality, e.g.:

>>> seq = madx.sequence.XYZ
>>> multipole = seq.expanded_elements[0]
>>> multipole.align_errors
AlignError(dx=-2.1754812916408347e-05, dy=-6.556356908596416e-06, 
ds=0.0, dphi=0.0, dtheta=0.0, dpsi=0.0, mrex=0.0, mrey=0.0, mredx=0.0, 
mredy=0.0, arex=0.0, arey=0.0, mscalx=0.0, mscaly=0.0)

>>> multipole.field_errors.dkn
[0.0,
 1.1472076071601915e-06,
 5.002980147857331e-06,
 0.013090083041168831,
 -0.19713806015162913,
 -73.91912412030126,
 -9.796705181638819,
 0.0,
(...)
 0.0]

2. possibly remove current approach with error table read out (more cumbersome approach, bloated functionality in PySixTrack)

[rdemaria]

I merge the PR for the time being, we will patch the alignment errors later...

[aoeftiger]

Thanks, I'll put them as an issue in the sixtrack/pysixtrack repo then to remind myself