Fixed #1111 Add branching logic for sdp

[NimaSarajpoor]

See #1111

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This is to address PR 4 as described in #1118 (comment). I've copied the corresponding notes below:

Notice that we haven't utilized ANY of the _X_sliding_dot_product anywhere in the code except for _njit_sliding_dot_product but that hasn't altered any of the code in a meaningful way as it was purely a refactor. Only at the very end in this PR do we start working on allowing the other _X_sliding_dot_product to be used. This is where we focus on the branching logic!

Note that instead of core.sliding_dot_product = sdp._sliding_dot_product, we can redefine core.sliding_dot_product so that it performs checks (e.g., check whether pyfftw is installed BEFORE callings sdp._sliding_dot_product). This way, sdp._sliding_dot_product can make assumptions that the user knows what they're doing while core.sliding_dot_product is slower but is super safe to use (hence it is public)!

[gitnotebooks]

Review these changes at https://app.gitnotebooks.com/stumpy-dev/stumpy/pull/1200

[NimaSarajpoor]

As discussed previously in this comment, I am planning to replace core.sliding_dot_product with something like this:

def sliding_dot_product(Q, T, boundaries):      
      m = len(Q)
      n = len(T)
      sdp = None
      for boundary in boundaries:
            (LB_m, UB_m), (LB_n, UB_n), sdp_func = boundary
             if LB_m <= m < UB_m and LB_n <= n < UB_n:
                  return sdp_func(Q, T)
           
      return sdp_func_default(Q, T)