raise valueerror if interpolated wavelengths are out of range and wri…

[jnoelke]

…te write documentation more generic

• add error handling if the wavelength for the get_value_for_wavelength function is smaller or larger than the provided min/max wavelengths
• There were some specific references to absorption. However, the class is more general and also handles scattering and anisotropy. We therefore wrote the documentation more generic.

Please check the following before creating the pull request (PR):

• Did you run automatic tests?
• Did you run manual tests?
• Is the code provided in the PR still backwards compatible to previous SIMPA versions?

Provide issue / feature request fixed by this PR

Fixes #221

[kdreher]

looks good