Fixes masked array issue with numba (fixes #322)

Numba v-0.57.0 is raising an error for masked arrays since there is no support for them. This causes an issue for Spell_length and First_spell. Since the mask is strict, i.e, stored before the computation (for any grid-cell containing a masked value) and applied after, the mask is not affecting the computations.

To fix the error I fill the masked values with False (the data is bool because of a threshold comparison), which also returns a ordinary array. The output is unchanged because of the strict masking.