Clean up internally Standard, Lagged Convolution, and Splined DLF for Hankel and Fourier transform.

A recent conversation with @sgkang from SimPEG reminded me of some chaotic things within empymod, whose roots lie in the way the code grew over time, and also how I learned over time. For instance, the original DLF functions are called fht (Fast Hankel Transform), as only the Hankel transform was considered, time-domain got added later. And over time different possibilities were added for Hankel DLF and Fourier DLF, the most recent is Standard Fourier DLF which was only added with empymod v1.5.2.

DLF implementation and their different optimisations

The methods that are possible with empymod for the Hankel and Fourier DLF are:

Standard DLF (No optimisation)
Lagged Convolution DLF (interpolation in output domain); spacing defined by filter
Splined DLF (interpolation in input domain), spacing defined by pts_per_dec.

Status quo

At the moment, this is differently implemented for Hankel and for Fourier:

Hankel DLF

standard: pts_per_dec=None
lagged: pts_per_dec=None, opt='spline'
splined: pts_per_dec>0, opt='spline'

Fourier DLF

standard: pts_per_dec<1
lagged: pts_per_dec=None
splined: pts_per_dec>0

Idea forward

I think a good implementation would be to have for both, Hankel and Fourier:

standard: pts_per_dec=0
lagged: pts_per_dec<0
splined: pts_per_dec>0

Therefore only having one parameter, and getting rid of opt='spline'.

This would mainly affect empymod.transform, and implementation is relatively easy. Functionality in empymod.utils could be extended to keep it backwards-compatible for all empymod.model-routines (including opt='spline'). However, the changes would be backwards incompatible for empymod.transform-functions. So it has to be done carefully to not break code which depend on these routines.

emsig / empymod