Simply benchmarks, release fix to numpy 1.17

[prisae]

Currently, the tests are fixed to numpy=1.17, because many benchmarks will fail for earlier versions.

Now, we don't need to run benchmarks all the way back. The history benchmarks are stored in
https://github.com/empymod/empymod-asv-v1/
and rendered on
https://emsig.xyz/empymod-asv-v1/

ToDo's in the near future:

• Release fix to numpy=1.17 (in config file);
• Simplify model and transform benchmarks (e.g., remove model.time and model.frequency tests).

[prisae]

Annotate it in the README.