Allow different astropy Table.read() reader options when accessing MTLs

[geordie666]

This PR updates the desitarget I/O code for MTLs to allow different reader options to be passed to the core Table.read() reader function.

Context:

• We originally fixed the reader for MTLs to ascii.basic, as this reader is ~3-4x faster than, e.g., ascii.ecsv.
  • The ascii.basic option will remain the default after this PR.
• The alt-MTL/mocks work is occasionally tripping on formatting errors that could be related to strict parsing using ascii.ecsv instead of ascii.basic.
  • This has never been a problem for the real MTLs, though.

This PR should address issue #818.

[geordie666]

Unit tests will fail in this branch, but have been fixed in #814.