Replace `np.float` with `np.float64` to address deprecation

[erguntiryaki]

np.float was deprecated in NumPy v1.20 and removed in v1.24. Using mgatk with the latest NumPy release causes an error, resulting in the absence of output files such as *.variant_stats.tsv.gz and *.cell_heteroplasmic_df.tsv.gz.