ENH: Changed type casting in regrid.

**NOT FOR MERGING**
This PR is up at this moment for discussion and is **not yet intended for merging** (test results have not been updated).

This PR:

1. Makes the resulting regrided array the same dtype as the source (rather than 64bit, which consumes twice as much memory for us here using 32bit data).
2. Defines the dtype of the source and target coordinate bounds to be operated as the highest **source** coordinate bound precision.  We should not define our regridded data as being defined on a more accurate grid than the source as it does not make sense to increase precision (which can currently occur if we were to define 64bit bounded coordinates on our target grid when we have 32bit bounds on our source for example).

- [ ] Agree on the statistical correctness of these changes.
- [ ] Update the test results to reflect these changes.