Benchmarked with asv

• 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.