Fix #65 Tetrahedron

[ezrayst]

Note: no regression test for tetraherdon yet

[codecov]

Codecov Report

Merging #66 into develop will increase coverage by 0.92%.
The diff coverage is 48.57%.

@@             Coverage Diff             @@
##           develop      #66      +/-   ##
===========================================
+ Coverage     30.2%   31.13%   +0.92%     
===========================================
  Files           20       20              
  Lines          864      941      +77     
===========================================
+ Hits           261      293      +32     
- Misses         603      648      +45

Impacted Files	Coverage Δ
include/gmsh.h	0% <ø> (ø)	⬆️
include/mesh.h	0% <ø> (ø)	⬆️
include/element.h	25% <ø> (ø)	⬆️
include/io.h	0% <ø> (ø)	⬆️
src/main.cc	0% <0%> (ø)	⬆️
src/io.cc	0% <0%> (ø)	⬆️
include/gmsh.tcc	0% <0%> (ø)	⬆️
tests/gmsh_test_2d.cc	3.7% <0%> (-0.07%)	⬇️
tests/volume_test.cc	100% <100%> (ø)	⬆️
src/element.cc	83.33% <100%> (+9.8%)	⬆️
... and 1 more

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