examples: Add diffpy.srfit examples

[cadenmyers13]

closes #4

tested each example by copying to cwd with cmi copy core and cmi copy nanoparticle-fit and everything ran as intended.

current cmi examples are:

$ cmi info examples

Examples:
---------
  core:
   - debye-waller
   - gaussianfit
   - intensityfit
   - linefit
   - readme
  pdf:
   - ch03NiModelling
   - ch05Fit2Phase
   - ch06RefineCrystalStructureGen
   - ch07StructuralPhaseTransitions
   - ch08NPRefinement
   - ch11ClusterXYZ
   - core-shell-NP

This also adds the new module diffpy.cmi.fit_tools where we house a lightweight optimization wrapper and a plotting function. in the future, we can add different optimizers to the optimization function, but for now it just has scipy.leastsq.

[cadenmyers13]

@sbillinge ready for review. fyi, srfit had a readme file to explain some of the examples. I added it here if the user is interested.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.63%. Comparing base (f6ce689) to head (1eb0de5).
⚠️ Report is 42 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   92.78%   93.63%   +0.85%     
==========================================
  Files           3        4       +1     
  Lines         194      220      +26     
==========================================
+ Hits          180      206      +26     
  Misses         14       14              

see 3 files with indirect coverage changes

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[cadenmyers13]

Added one thing to docs. ready for review

[cadenmyers13]

@sbillinge please see the other PR here #94 for more discussion on this test.