docstrings and instances added

[gpap-gpap]

@berland pinging you because I am not sure an automated email gets sent with draft PRs

Feel I should contribute to this as I was using it today to test some code I was writing internally. I made four changes:

• Used type hints at base level of methods (i.e. not in submethods), and declarations involiving Material class. (this makes use of future import for old versions which may be a bummer for the devs so happy to roll back)
• Rewrote the docstrings for the methods adding detail wrt to previous docstring and some accuracy as to what is being returned (instance of a class with bound moduli rather than a "gives a bound on moduli") as well as examples. If there is a style guide that new docstrings violate happy to roll back
• generate class instance and only return it if class instantiated successfully. Not sure this can ever fail the way things are written now, but I don't see how it can hurt. Again, if this contradicts with internal style guide happy to roll back.
• I also wrote a check so that hashin_shtrikman_walpole bound argument is checked to be either of lower and upper (it was written so that any string other than lower would give the upper bound. This could be breaking if someone somewhere has misspelled code!)

I have identified some inconsistency regarding the types of attributes in the Material class, namely generating a material with hashin_shtrikman_average has float attributes which I feel is discouraged over array of a single float. But this doesn't seem enforced so I haven't done anything about that.

[eivindjahren]

Sorry for taking so long to get to this, this unfortunately got lost in our notifications. I will get on this as soon as I can.

[eivindjahren]

I rolled back the reraise of ValueError as you mentioned, I agree that this handling is not great, but I would need to rethink it in more detail.

The inconsistency is just that float and np.vector[float] are used interchangeably throughout. Again, something to make more consistent.

[eivindjahren]

Thank you so much for your contribution, sorry for taking so long to get to it.