MNT: Significant change to the gridder api

- Broke out the bounds into their own parameters
- Added a return value of the bounds that were used on the gridder
MNT: Added gridder defaults to core.py
MNT: Fixed tests based on update to gridder api
MNT: Proposed name change of process_grid to grid3d
DOC: Updated docs based on devteam feedback
MNT: Collapsed if block
MNT: Added new file to api changes
Merge pull request #114 from ericdill/api_gridder

Significant change to the gridder api