Update fitters and add test that weights behave as expected

[henrikjacobsenfys]

Closes #204

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.11%. Comparing base (ed35c99) to head (a99f068).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #216   +/-   ##
========================================
  Coverage    81.11%   81.11%           
========================================
  Files           52       52           
  Lines         4274     4274           
  Branches       740      740           
========================================
  Hits          3467     3467           
  Misses         627      627           
  Partials       180      180           

Flag	Coverage Δ
unittests	81.11% <100.00%> (ø)

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Files with missing lines	Coverage Δ
.../easyscience/fitting/minimizers/minimizer_bumps.py	88.97% <100.00%> (ø)
...rc/easyscience/fitting/minimizers/minimizer_dfo.py	92.91% <100.00%> (ø)
.../easyscience/fitting/minimizers/minimizer_lmfit.py	92.17% <100.00%> (ø)