Abstract base class for SphericalHarmonicDefiniton and -Audio

[mberz]

Changes proposed in this pull request:

• Create an abstract base class for SphericalHarmonicDefinition for more flexible re-use in case n_max is not required or a getter only property
• New ABC can be used as parent in Add SphericalHarmonicsAudio classes #166
• Child classes which should not allow setting properties will need to overload the respective setter property.

[f-brinkmann]

Thanks, I found one redundancy that can maybe be removed (see comment) and have two questions:

Is testing not implemented because that done as part of the SH class?

Child classes which should not allow setting properties will need to overload the respective setter property.

Would it make sense to have two abstact base clases? One containing the properties and on the getter. This might reduce redundancy in case multiple derived classes only require getters.

[mberz]

Would it make sense to have two abstact base clases? One containing the properties and on the getter. This might reduce redundancy in case multiple derived classes only require getters.

I've thought about it, but then it would require re-writing the entire setter and checking logic to the state we had before including n_max in the base class if I remember correctly. At this point I don't think it makes sense to go back on that again.

[f-brinkmann]

Thanks for checking!

[sikersten]

Thank you! I only have two minor comments, otherwise approved.