All file open functions should be changed to use the open pathlib method

[tjmills]

Since we are using pathlib.Path everywhere, we should use the bound pathlib.Path.open method to open files rather than the base pytohn open method.

The base python open method throws an error on pathlib.Path objects

TypeError: expected str, bytes or os.PathLike object, not PosixPath

[tjmills]

dartclasses.py uses strings for filepaths, this should be changed to pathlib.Path objects.

e.g. https://github.com/NCAR/wrf_hydro_py/blob/master/wrfhydropy/core/dartclasses.py#L92