Make some changes to cohorts to keep Analysis compatible w/ Experience

[MaryanMorel]

Add cache method to Cohort as it is now used by Experience

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@75c0291). Click here to learn what that means.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##             develop      #33   +/-   ##
==========================================
  Coverage           ?   85.22%           
==========================================
  Files              ?       11           
  Lines              ?      636           
  Branches           ?        0           
==========================================
  Hits               ?      542           
  Misses             ?       94           
  Partials           ?        0           

Impacted Files	Coverage Δ
scalpel/core/cohort.py	86.01% <20.00%> (ø)
scalpel/core/decorators.py	34.78% <0.00%> (ø)
scalpel/core/util.py	100.00% <0.00%> (ø)
scalpel/flattening/flat_table_collection.py	100.00% <0.00%> (ø)
scalpel/core/cohort_flow.py	92.85% <0.00%> (ø)
scalpel/core/cohort_collection.py	76.28% <0.00%> (ø)
scalpel/core/io.py	71.42% <0.00%> (ø)
scalpel/flattening/table.py	91.89% <0.00%> (ø)
scalpel/flattening/flat_table.py	98.24% <0.00%> (ø)
scalpel/core/descriptors.py	0.00% <0.00%> (ø)
... and 2 more

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[ysebiat]

LGTM.

[MaryanMorel]

@dsun0720 or @kevtokev could you do a quick review? This PR was already approved, I just rebased and added two tests to maintain the coverage.