`dlftools` (helper fcts for fdesign for very expensive kernels)

[prisae]

The branch https://github.com/emsig/empymod/tree/dlftools contains routines that were developed for

Grayver, A., A. Kuvshinov, and D. Werthmüller, 2021, Time‐Domain Modeling of Three‐Dimensional Earth's and Planetary Electromagnetic Induction Effect in Ground and Satellite Observations: -- Journal of Geophysical Research: Space Physics 126(3), e2020JA028672; DOI: 10.1029/2020JA028672.

These routines are useful when designing digital linear filters with very expensive kernels. They allow, amongst other things, to adapt the wanted times re-using existing frequencies.

These tools need

• cleaning up,
• proper integration into fdesign,
• documention, and
• tests.