Added `regridding.fill()` function to fill in missing data using inte…

…rpolation (#3)

Added `regridding.fill()` function to fill in missing data using interpolation.

Fixed bug in `regridding.regrid_from_weights()` where zeros were bein…

…g returned if the input arrays were not contiguous.

Updated codecov badge in the readme.

Added example to documentation for `regridding.regrid()`.

Renamed project from `interp` to `regridding` since `interp` wasn't a…

…vailable on PyPI.

Signed-off-by: Roy Smart <[email protected]>