Fixes #215

Changes proposed in this pull request:

sktree/tree/manifold/_morf_splitter.pyx

• Rewrote the logic for sampling indices of patches (I couldn't really understand the discontiguous loops)
  1. Instead of working in raveled index space, we work in the unraveled index space
  2. For each dimension, if dim_contiguous, then we sample indices sequentially from top_left_seed to the top_left_seed+patch_dim. If not, then we randomly sample indices based on patch_dim.
  3. We take the cartesian product of all the indices across dimensions, then ravel each element in the product.
• Some of the datatypes changed to using vectors because I was getting Python object/GIL errors on compilation when trying to use arrays and vector[vectors] interchangeably.
• Removed some uses of class attributes that are rewritten constantly

sktree/tree/_utils.pyx

• Added cartesian product functions
• Changed unravel_index_cython to return rather than inplace replacement of one of the inputs
• Changed intp_t[:] to vector[intp_t] for inputs/outputs for some functions for compatibility with changes to _morf_splitter.pyx.

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