Bugfix in reference_grid(TET)

[pmartorell]

Dear @fverdugo ,
this an small bugfix on the changes introduced in #704

The new tests also checks that only the boundary facets have one single cell around.

[codecov-commenter]

Codecov Report

Merging #734 (eb8fbab) into master (b25cf8b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #734   +/-   ##
=======================================
  Coverage   88.39%   88.39%           
=======================================
  Files         132      132           
  Lines       15454    15454           
=======================================
  Hits        13660    13660           
  Misses       1794     1794           

Impacted Files	Coverage Δ
src/Geometry/Grids.jl	87.50% <100.00%> (ø)

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[fverdugo]

@pmartorell Thanks for the fix. Have a nice new year's eve!