Hotfix gradient with cache intermediates

[thangleiter]

If calling infidelity and then infidelity_derivative, the control matrix is cached during the first call, but not the intermediates. This leads to an exception when gradient.calculate_control_matrix_derivative_from_scratch() tries to access the intermediates (since PulseSequence.get_filter_function_derivative() passes them along).

[codecov]

Codecov Report

Merging #58 (de02256) into master (cde8258) will increase coverage by 0.22%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   97.04%   97.27%   +0.22%     
==========================================
  Files           9        9              
  Lines        2203     2203              
  Branches      499      499              
==========================================
+ Hits         2138     2143       +5     
+ Misses         29       26       -3     
+ Partials       36       34       -2     

Impacted Files	Coverage Δ
filter_functions/numeric.py	94.82% <66.66%> (ø)
filter_functions/gradient.py	100.00% <100.00%> (+4.67%)	⬆️

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