empymod/empymod: empymod v1.7.2

Benchmarks: empymod has now a benchmark suite, see empymod/asv.

Fixed a bug in bipole for time-domain responses with several receivers or sources with different depths (Simply failed, as wrong dimension was provided to tem).

Small improvements:

• Various simplifications or cleaning of the code base.
• Small change (for speed) in check if kernels are empty in transform.dlfand transform.qwe.