Passband general improvements

[OliviaLynn]

• We now have LSST transmission tables stored in our test data directory (and a passband_dir fixture defined in our conftest); it would be nice to rewrite some of our unit tests to use these tables rather than toy data

• Use astropy units in Passband code

Should we just use astropy units here so we can use all their conversions?

Originally posted by @jeremykubica in #111 (comment)

• Refactor Passband constructor

I think it is fine to keep this as-is, but an alternative approach that might be more modular approach might be to have the constructor take a table and then to add two class methods:

@classmethod
def from_file(cls, table_path: str, ...):
   load the file
   ...
   return Passband(table, ...)

@classmethod
def from_url(cls, table_url: str, force_download: bool, ...):
    do the download
    ...
    return Passband(table, ...)

Originally posted by @jeremykubica in #111 (comment)