New function to combine feature dataframes

[freemansw1]

This should address #17 (!) and provides a function for combining multiple, independently identified feature dataframes into a single dataframe appropriate for tracking.

• Have you followed our guidelines in CONTRIBUTING.md?
• Have you self-reviewed your code and corrected any misspellings?
• Have you written documentation that is easy to understand?
• Have you written descriptive commit messages?
• Have you added NumPy docstrings for newly added functions?
• Have you formatted your code using black?
• If you have introduced a new functionality, have you added adequate unit tests?
• Have all tests passed in your local clone?
• If you have introduced a new functionality, have you added an example notebook?
• Have you kept your pull request small and limited so that it is easy to review?
• Have the newest changes from this branch been merged?

[freemansw1]

I should note that the original idea for this came from some code that @JuliaKukulies posted that I found a couple years ago on #17.

[codecov]

Codecov Report

Base: 35.06% // Head: 35.58% // Increases project coverage by +0.52% 🎉

Coverage data is based on head (dedefdb) compared to base (dfe3b38).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@              Coverage Diff              @@
##           RC_v1.4.0     #186      +/-   ##
=============================================
+ Coverage      35.06%   35.58%   +0.52%     
=============================================
  Files             10       10              
  Lines           2099     2116      +17     
=============================================
+ Hits             736      753      +17     
  Misses          1363     1363              

Flag	Coverage Δ
unittests	35.58% <100.00%> (+0.52%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tobac/utils.py	50.40% <100.00%> (+3.64%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[snilsn]

Good addition, @freemansw1 ! The function is very well thought out, and I have tested it by splitting the test data sets. It's working flawlessly. All of my comments are just some minor details, therefore I'm approving this PR.

[JuliaKukulies]

Perfect, thanks for taking care of this @freemansw1! Your code works well and the documentation is clear and helpful! I have only a small question. Otherwise, nothing to add.

[freemansw1]

Ok, since I've added new functionality, will wait for @JuliaKukulies and @snilsn to confirm (and CI to pass) before merging. Thanks to both of you for your very quick reviews!

[snilsn]

No concerns from my side @freemansw1

[JuliaKukulies]

Not from my side either. Please go ahead and merge @freemansw1

And thank you very much for the addition, I find it useful!