MRMS precip_type handling

[oznogr]

When running CHIMP with 'mrms_w_type', the precip_type tensor is not handled properly.

In RefernceDataset.load_sample:

              if not np.issubdtype(y_t.dtype, np.floating):
                    y_t = y_t.astype(np.int64)

changes the precip_type y_t target to int64 and then:

            if invalid is not None:
                y_t[invalid] = np.nan

throws an error because y_t is int64 and np.nan is float.