Hook for Cole-Cole IP and similar

• model.bipole, model.dipole, and model.analytical have now a hook which users can exploit to insert their own calculation of etaH, etaV, zetaH, and zetaV. This can be used, for instance, to model a Cole-Cole IP survey. See the manual or the example-notebooks for more information.

• model.wavenumber renamed to model.dipole_k to avoid name clash with kernel.wavenumber. For now model.wavenumber continues to exist, but raises a depreciation warning.

• xdirect default value changed from True to False.

• Possibility to provide interpolated points (int_pts) to transform.dlf.

The following changes are backwards incompatible if you directly used transform.fht, transform.hqwe, or transform.hquad. Nothing changes for the user-facing routines in model:

• empymod.fem now passes factAng to empymod.transform, not angle; this saves some time if looped over offsets or frequencies, as it is not repeatedly calculated within empymod.transform.

• Use get_spline_values in empymod.fem for Hankel DLF, instead of in empymod.fht. Gives a speed-up if looped over offsets or frequencies. Should be in utils, but that would be heavily backwards incompatible. Move there in version 2.0.