add penalties/constraints for positive and negative functions

[silviobe]

Add constraints for function to be positive or negative valued.

[codecov]

Codecov Report

Merging #281 into master will increase coverage by 0.04%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #281      +/-   ##
==========================================
+ Coverage   95.21%   95.26%   +0.04%     
==========================================
  Files          22       22              
  Lines        3178     3252      +74     
==========================================
+ Hits         3026     3098      +72     
- Misses        152      154       +2     

Impacted Files	Coverage Δ
pygam/penalties.py	91.95% <83.33%> (-1.38%)	⬇️
pygam/tests/test_penalties.py	100.00% <100.00%> (ø)
pygam/tests/test_terms.py	100.00% <0.00%> (ø)
pygam/pygam.py	94.80% <0.00%> (+<0.01%)	⬆️
pygam/tests/test_utils.py	96.55% <0.00%> (+0.04%)	⬆️
pygam/callbacks.py	95.23% <0.00%> (+0.07%)	⬆️
pygam/terms.py	94.60% <0.00%> (+0.19%)	⬆️
pygam/distributions.py	88.88% <0.00%> (+0.48%)	⬆️
pygam/tests/conftest.py	98.21% <0.00%> (+0.65%)	⬆️

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