Add FileContainer search_intersection method

[veni-vidi-vici-dormivi]

Here is a method that enables searching for intersecting values of a certain key along all values of another key. The specific use case here is: "Which scenarios and members are available for both variables tas and hfds" for example.

The usage would be to find all available scenarios and members for both variables and then search the resulting FileContainer for intersecting values along scenario_member. In this case search_key = variable and intersect_key = scenario_member. I chose this approach because I felt it relatively straight forward, more than implementing it in FileFinder.

I am not very happy with the names and my explanation in the docstring, but at least the example should make it quite clear.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Flag	Coverage Δ
unittests	99.71% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
filefisher/_filefinder.py	100.00% <100.00%> (ø)

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