Reinstantiation of CellVolumes, FaceNormals etc.

[chmerdon]

• added update! function to trigger reinstationation of cell components
• CellVolumes, FaceVolumes etc. and FaceNormals and EdgeTangents now keep existing arrays

Example:
update!(grid, CellVolumes)
checks if CellVolumes exists and only then triggers instantiate(grid, CellVolumes) which then keeps the existing array and just updates the values

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 73.63%. Comparing base (06a2f11) to head (edc1f93).
Report is 7 commits behind head on master.

Files	Patch %	Lines
src/derived.jl	36.36%	7 Missing ⚠️
src/extendablegrid.jl	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   73.01%   73.63%   +0.62%     
==========================================
  Files          27       27              
  Lines        4239     4244       +5     
==========================================
+ Hits         3095     3125      +30     
+ Misses       1144     1119      -25     

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