More Grid Docs, mostly related to API Reference

[koehlerson]

Continued the grid docs, but this time focusing on the API reference. I tried to include everything which is relevant for subtyping AbstractGrid. Hopefully, it will be easier now to understand how Grid works, but also how one can subtype AbstractGrid and define a suitable interface for JuAFEM

[codecov-io]

Codecov Report

Merging #306 (47f4906) into master (40e515b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #306   +/-   ##
=======================================
  Coverage   85.84%   85.84%           
=======================================
  Files          21       21           
  Lines        1999     1999           
=======================================
  Hits         1716     1716           
  Misses        283      283           

Impacted Files	Coverage Δ
src/Grid/grid.jl	83.72% <ø> (ø)

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[koehlerson]

I still have to add in the utility function sections that these should be dispatched on a custom AbstractGrid subtype in case someone wants to use their own grid format. What do you think, should I append it below, put it at the top? I think right now I'd put it on top of the section and immediately afterwards a Documenter @index block listing all neccessary functions.

[codecov-commenter]

Codecov Report

Merging #306 (1ad1c1c) into master (bd44c89) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #306   +/-   ##
=======================================
  Coverage   91.03%   91.03%           
=======================================
  Files          22       22           
  Lines        2855     2855           
=======================================
  Hits         2599     2599           
  Misses        256      256           

Impacted Files	Coverage Δ
src/Grid/grid.jl	86.62% <100.00%> (ø)

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[termi-official]

First big round. I think a major issue is that we do not specify what the interface does, but leak implementation details of the default grid implementation. We should focus more on what is happening conceptually.

[koehlerson]

ready for review again :)

[termi-official]

Got two more minor points and some missing dots. If this is fixed the PR should be ready.

[termi-official]

LGTM

[kimauth]

Looks really good to me!