add constant signal class

[wenboown]

Thanks for making the library available. It is very helpful to me. I encountered a situation where I need a constant signal so I just created a class for it.

[codecov-io]

Codecov Report

Merging #10 into master will decrease coverage by 0.11%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   48.77%   48.65%   -0.12%     
==========================================
  Files          17       18       +1     
  Lines         285      298      +13     
==========================================
+ Hits          139      145       +6     
- Misses        146      153       +7

Impacted Files	Coverage Δ
timesynth/signals/constant.py	46.15% <46.15%> (ø)

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[darigovresearch]

@wenboown thanks for taking the time to make a pull request, are you able to make some tests to manage the coverage of the automatic message above?

Do the maintainers need anything else to merge this pull request?