Improve `optimize`

[prisae]

Since v0.12.0 there is the optimize module. However, this is still considered experimental, and many things have to be improved. This is an issue to keep track of that:

• Taking into account std
• Magnetic receivers
• #228
• #229
• #235

Also, optimize.gradient should be made stand-alone, so it can be called from other packages.

[prisae]

• Depth weighting and regularization will not happen in emg3d, these are things for inversion frameworks.
• All the other points are tracked in separate issues
• v1.2.1 removed the optimize module entirely, hence, closing this issue