Correct scale for the exact TransverseWakeFunction in CircularResistiveWall.
Thanks to Keon Hee Kim for providing the correction factor.

The approximated longitudinal wake function in CircularResistiveWall correctly takes into account the length

WakePotential -> add method to plot wake potential for a perfect gaussian bunch
utilities -> new function gaussian_bunch which gives the gaussian bunch profile
utilities -> read_IW2D now also reads wake function from IW2D
utilities -> new function read_IW2D_folder which reads the full results folder of IW2D into a WakeField element.
resistive_wall -> change the sign of the wake functions to agree with the code convention

The exact wake function for the transverse wake function is wrong (from paper) ! Need to find a better expression.

Remove tools.py to avoid circular import problem with wakefield.py -> merged into wakefield.py
Remove machines folder to save machine data in a seperate git instance
Rework import so the code can be used as independant module mbtrack2

Add a ImpedanceModel object which derives from Element and is used to sum up different Wakefield elements
Fix uppercase module in resistive_wall.py

To be PEP8 complient:
Lowercase folders
Extended docstrings
Rename and divide some files