Add conditional to set _mv variable to np.uint64 data type

[robgpita]

Allow the processing of large rasters, shaped at (63708, 79780), for Flwdir object.

Issue addressed

Closes #46

Explanation

As described in issue #46, when large rasters are used, an IndexError was thrown from underlying numba code.

The upstream_count function was the cause of the Indexing Error, due to an incorrect value set in the _mv variable when using a np.uint64 data type.

Checklist

• [x] Updated tests or added new tests
• [x] Branch is up to date with main
• [ ] Updated documentation if needed
• [x] Updated CHANGELOG.rst if needed

Additional Notes (optional)

The setting of the np.uint64 dtype in pyflwdir.py was not accounted for, and caused non-correct values in the _mv variable, when assigned in core.py. This was causing the C pointer references to not line up with the signed integer type that was being used in python. See numpy docs for np.intp()

The downstream effects