Feature: Adding slippage functionality to SignalReturnRelations

[Magnus167]

Adding a method apply_slip() to SignalReturnRelations.
The function is a replica of CategoryRelation.apply_slip().

Closes #802

Once the functionality is verified independently for both these functions, they'll be merged to a common utility/DF tool.

[codecov]

Codecov Report

Patch coverage: 83.87% and project coverage change: +0.09 🎉

Comparison is base (d129d0e) 73.74% compared to head (f915054) 73.83%.

❗ Current head f915054 differs from pull request most recent head c35f206. Consider uploading reports for the commit c35f206 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #819      +/-   ##
===========================================
+ Coverage    73.74%   73.83%   +0.09%     
===========================================
  Files           31       31              
  Lines         3671     3696      +25     
===========================================
+ Hits          2707     2729      +22     
- Misses         964      967       +3     

Impacted Files	Coverage Δ
macrosynergy/panel/view_ranges.py	12.69% <0.00%> (ø)
macrosynergy/panel/view_timelines.py	91.07% <75.00%> (ø)
macrosynergy/signal/signal_return.py	73.07% <86.95%> (+1.37%)	⬆️
macrosynergy/panel/category_relations.py	40.23% <100.00%> (+0.70%)	⬆️

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