Add subclass for decompressed arrays #464

Following this comment, add a thinly wrapped native Blosc2 class for decompressed arrays, which are currently just plain NumPy arrays. Would have to support certain additional operations and comparisons (==, __array_namespace__()), but it will inherit most of the useful machinery directly from NumPy.

Would have to change np.empty references for loading slices etc. from C functions in the code base, possibly some other things.