C Interface

[jacobwilliams]

Use iso_c_binding to make a C-style interface to the routines now in a module. See also #14.

[jacobwilliams]

Aside: https://github.com/devernay/cminpack. Something else that can be studied.

Our goal should be to show a better way, that doesn't involved converting everything to C/C++.