calc_kde: allow for arbitrary number of KDE parameters and log vs non-log choice

[tdent]

Currently pycbc_template_kde_calc only calculates a KDE over the parameters ln(mchirp), eta and chieff, as these are hard-coded. To enable investigation of the effect of different ways of making the KDE, we would like to have the following options / abilities:

• Any number of parameters can be used
• Parameters can be any of the named parameters in pycbc/tmpltbank/bank_conversions.py
• User can choose to either use the parameter as-is to make the KDE or use its natural log There is already an example of some syntax for getting similar options in the fit_sngls multiparam code, see https://github.com/gwastro/pycbc/blob/master/bin/all_sky_search/pycbc_fit_sngls_over_multiparam#L163