Feature/analyticSC

[aoeftiger]

Adds analytic space charge module, from docstring:

Analytic transverse space charge fields based on
3D Gaussian distribution (Bassetti-Erskine formula).

Can track with respect to the bunch centroid and can cumulatively
update transverse bunch size every n steps.

As opposed to self-consistent longitudinal line charge density
through slicing in TransverseGaussianSpaceCharge class, this
AnalyticTransverseGaussianSC class assumes a longitudinal Gaussian
line charge density based on a given RMS sigma_z.

[aoeftiger]

--> possible improvement: rolling average for the adaptive cumulative beam size. (not yet implemented)