replace combine with itertools product

[dswah]

for large search-spaces, gridsearch can easily exhaust all available memory before fitting a single model.
This is because the method tries to pre-determine the entire search space in memory.

we could instead use a generator like itertools.product

this fixes #242

TO-DO

• tests
• remove all references to pygam.utils.combine

[codecov]

Codecov Report

Merging #253 into master will increase coverage by 0.01%.
The diff coverage is 91.66%.

@@            Coverage Diff            @@
##           master    #253      +/-   ##
=========================================
+ Coverage   95.18%   95.2%   +0.01%     
=========================================
  Files          22      22              
  Lines        3178    3169       -9     
=========================================
- Hits         3025    3017       -8     
+ Misses        153     152       -1

Impacted Files	Coverage Δ
pygam/utils.py	87.61% <ø> (-0.12%)	⬇️
pygam/pygam.py	94.81% <91.66%> (+0.01%)	⬆️

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