Displacement-strain-planet

**Improvements:**
_SH_deriv_store_
-Now saves the spherical harmonic functions 'y_lm', which can be used in Displacement_strains. This moderately increases the strain computation speed, but increase the size of the file saved in SH_deriv_store.
-Add "compressed" optional argument to allow saving with np.savez_compressed which reduces the file size by about a factor 2. Since we do compression, this takes slightly longer to load and save the Ylms.

_Displacement_strains_
-It is not possible anymore to compute SH derivatives on the fly (with precomp==0) as this was very slow and likely not worth it. The "precomp" argument is now gone.
-The function has been rewritten in vector form and making use of the numpy.einsum function. This speeds up the function by a factor of about 3.

**Error correction:**
_Plt_tecto_Mars_
-Correct a typo in the function plotting the Knapmeyer dataset. We were only iterating over half of the data.

Minor update to fasten examples/Mars_SouthPolarCap_displacement.py for publication

compilation: added the cmcrameri python package requirement
examples: added Venus example
code: added a "filter_in" argument for the user to input any given filter rather than using those built in
docs: updated

Corrected a typo in the stress calculation (theta phi component)

Improved the SH_deriv routine for faster computations

Added an example file Mars_SouthPolarCap_displacement.py that demonstrates how to calculate iteratively the flexure underneath the south polar cap of Mars as a function of elastic thickness and ice density.

Release for Zenodo.

Rewrote SH_deriv_store and SH_deriv for faster computations of spherical harmonic derivatives.

Added the possibility to not save the files in SH_deriv_store with the optional boolean argument 'save'

Updated webdoc.