Add GAP code constructions

[perlinm]

(also make the small_groups tests more robust)

You can now do something like

import qldpc
code = qldpc.codes.ClassicalCode.from_name("CordaroWagnerCode(5)")
print(code.matrix)

TODO:

• [x] Fail gracefully when a named code isn't found
• [x] Retrieve base field from the GAP code
• [x] Save results to a local disk cache