Warns and errors

[alhom]

Going through logging.info calls etc. to have actual errors and warnings raised instead of printing "ERROR" to log and causing problems in downstream.

Plan:
logging.info("WARNING something suspicious") -> warnings.warn("something suspicious") or logging.warn("something suspicious") if the warning bears repeating
logging.info("ERROR a bad thing") -> raise ADescriptiveException("a bad thing")
if a != b: logging.info("This is weird, check pls") -> assert a == b

Going through the files changed in #269, since that's a handy list. Going through file-by-file.

[alhom]

@vetarvus, would you agree with the changes to non_maxwellianity? If there is no B available for some cell, I don't think there will be for other cells either, and then it's no use to continue calculation with a bi-maxwellian anyway, no?

[vetarvus]

Non-Maxwellianity looks good to me!