Adding the taper RW impedance computation

The taper resistive-wall impedance is added, in a similar way as what is done for the geometric part.

It uses the simple, approximated RW formula newly implemented in PyWIT ( MR !21 ) and its trapz integration to get a taper RW impedance, from PyWIT MR !22 , or a two-layer round chamber computation with Yokoya factors if there is coating on the taper. Hence IW2D is now needed.

It works by adding the layers keyword in taper_data.json for each taper, containing layers (formatted as a list of dict in the usual way). Also, flat parts in the list of tapers were added (they contribute only to the resistive-wall part of the taper impedance).

Note that the functions to compute the taper RW impedance are added in collimator.py - some of them (especially the one creating a round impedance from IW2D) might need to be migrated into PyWIT later on (when the python interface between PyWIT and IW2D will be fully functional).

Note also that for this part we assume the computation is short enough hence no launch/retrieve mechanism, nor the keeping of any input/output file, is implemented.

The corresponding wake is not computed at this stage - this will also require more work (on IW2D interface, or using neffint to obtain the full wake).

As for the geometric part, the taper RW impedance is computed by default, but a parameter of LHCModel can remove it (compute_taper_RW_impedance_collimators).

In addition:

• all settings files updated,
• .gitignore added (ignoring __pycache__, *.egg-info and various folders created in examples),
• bug fixed in LHCModel (an error was raised when bpms_broadband_contributions_parameters_filename or tapers_broadband_contributions_parameters_filename, now the corresponding impedance is just not taken into account),
• new material cu_hipims (with conductivity 30 MS/m) added,
• better handling of TCTP mode (utils.py updated).

Note that tapers still have to be double checked (now essentially we are still based on previous taper types and the work done by S. Antipov and E. Carideo). See issue #27 .