Implement npz2fits_method

[sk8forether]

This method should create a psrfits file, including headers, from a .npz file. The .npz file should contain a numpy array and the 2 headers as dictionaries, just as is created in the fits2npz_method.

[sk8forether]

@adambrazier I created this issue so we can keep discussing implementation here as needed. @swCornell Do you think that you can begin implementing this? I can create a basic method file to get you started if you need it, and you can fill in the conversion. Remember we want the conversion to work both ways: fits to npz and npz to fits, so the headers will have to be set up correctly and checked.