Make discretize a dependency

[prisae]

Currently, discretize is not a dependency, and all could be done with utils.TensorMesh. However, this leads sometimes to confusion, and the gain is little. The main reasons why discretize has not been made a dependency are removed:

• dependencies:
  • matplotlib is no longer a hard dependency
  • pymatsolver is no longer a dependency (Remove pymatsolver simpeg/discretize#193)
• Installable via conda
• Can be installed without prior installing numpy

So I think it is time to go all-in and make it a hard dependency (adjust docs and website) as soon as all boxes above are ticked and a release happened (pin the version then); utils.TensorMesh should then only be used internally.

I think the best would be to do in emg3d.meshes

from discretize import TensorMesh

and then rename the meshes.TensorMesh to meshes._TensorMesh, for internal use of the solver only.

Adjust accordingly:

• emg3d.meshes
• emg3d.io save/load
• example-gallery
• website
• REAMDE
• install instructions in docs

[prisae]

If so, replace emg3d.njitted.volume_average with discretize.utils.interputils.volume_average.

[prisae]

Will be closed by #109

[prisae]

Soft dependency.