Integer type for data indexing

[mchalela]

GriSPy/grispy/core.py

Line 370 in ee67826

k_digit = np.zeros(data.shape, dtype=int)

Right now the default int type for the data is int64 (in most cases). This may be optimized if we choose the int size according to the length of the input data. For example, indices of int32 are sufficient for a length of 10**9 in the input data. This can save some memory for large dimensions.