Remove explicit documention of fields in CellCache

[KnutAM]

• Better to use the functions, e.g. getnodes, as these also work with FacetCache.
• In PRs such as WIP: Add cell to CellCache #1060, not documented increases flexibility of changing representation (when cell is stored, no need to store the nodes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.71%. Comparing base (4b8c233) to head (a3c2ca1).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1079   +/-   ##
=======================================
  Coverage   93.71%   93.71%           
=======================================
  Files          39       39           
  Lines        6042     6042           
=======================================
  Hits         5662     5662           
  Misses        380      380           

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