Remove dependency on Numba

[matt-graham]

Currently Numba is only used to implement the localisation functions in src/dapy/utils/localisation.py and a utility function for the partition of unity implementation in src/dapy/utils/pou.py. Both would be relatively simple to translate to Cython to avoid requiring Numba as a dependency and to be more in line with the use of Cython elsewhere in the package.