extract_data input_module kwarg accepts strings

[jagilley]

I think it makes sense to modify extract_data to accept strings as the canonical input_module kwarg, as it saves the user from having to instantiate their own input_module. (Especially because we have the nice input_mapping dict just a few lines earlier!) This change would default the input_module kwarg to a string, but still maintain the legacy API because of the if clause on line 78 of my fork.

[m3nu]

Good to merge this, but docs should be updated too.