[MERGE] Tensor constraints

[dswah]

• [x] tensor terms should be able to have constraints
• [x] model with constraints AND unconstrained tensor terms should build
• [x] constraints should be composable
• [x] penalties should be composed correctly
• [x] tests

[codecov[bot]]

Codecov Report

Merging #225 into master will increase coverage by 0.15%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   94.86%   95.02%   +0.15%     
==========================================
  Files          22       22              
  Lines        3056     3113      +57     
==========================================
+ Hits         2899     2958      +59     
+ Misses        157      155       -2

Impacted Files	Coverage Δ
pygam/tests/test_terms.py	100% <100%> (ø)	:arrow_up:
pygam/terms.py	94.06% <100%> (+0.45%)	:arrow_up:
pygam/pygam.py	94.89% <0%> (+0.12%)	:arrow_up:

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