[ENH] - Add formula to Mode

[TomDonoghue]

This adds a formula attribute to the Mode object, so that a Mode definition can store a latex version of the mathematical formula that is fit.

Related / additional edits done here:

• Writes out the formulas for each implementation in specparam/modes/funcs
• Updates the naming of the aperiodic fit functions

ToDo:

• figure out / fix writing out formula for the skewed Gaussian

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.65%. Comparing base (32a28a8) to head (955c019).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
+ Coverage   97.63%   97.65%   +0.02%     
==========================================
  Files         160      160              
  Lines        6710     6740      +30     
==========================================
+ Hits         6551     6582      +31     
+ Misses        159      158       -1     

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