Improve error messages for missing keywords

[hamogu]

Currently, that just raises a KeyError at some point, but given the complex inheritance structure, it's easy to loose track of which keywords are required and which ones are optional.
Ideas:

• Compile list of required and optional keywords. This probably needs a class level variable that hold this info.
• It can also hold docs - can be added to the class docstring on generate with a metaclass.
• The top of every __init__ could have a check that prints the help for missing keywords - a metaclass might help again.

I've heard of "traits" at some point. Need to investigate if that already provides this functionality.

[hamogu]

Similarly, add warnings for process_photons runs:

• if photons is empty (I got a "col missing" error, because I processed an empty table, so the process_photons added that column because it was never needed).
• if photons is so long / big that memory problems might arise
• if there are nans in pos, dir (in other columns, nan might mean "not applicable")?
• ...

Could put these tests into __call__, so they would run only then and internally I continue to call process_photons. Would miss some warnings, but also save time.