Avoid seg fault for 3D extrusion without a MODEL block

[andrewwinters5000]

This is a bug fix that closes #48 . A model block is only required for the SWEEP type of extrusion. For simple extrusions or rotations the model block is not required. So, I added a check if the model block is ASSOCIATED to avoid referencing a NULL pointer.

[codecov[bot]]

Codecov Report

Merging #59 (8f9fa2c) into main (fd9043d) will increase coverage by 0.34%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   73.97%   74.31%   +0.34%     
==========================================
  Files          68       68              
  Lines       10259    10259              
==========================================
+ Hits         7589     7624      +35     
+ Misses       2670     2635      -35     

Flag	Coverage Δ
unittests	74.31% <66.66%> (+0.34%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Source/HOHQMesh.f90	65.00% <ø> (-0.58%)	:arrow_down:
Source/3DSource/3DMeshController.f90	93.13% <66.66%> (-0.87%)	:arrow_down:

... and 2 files with indirect coverage changes

[andrewwinters5000]

Unfortunately, this drops the codecov by a fraction of a %.

That is okay from my perspective. A bug fix plus a new meaningful error message is more important.

[DavidAKopriva]

When merged with the PR53, the total should go up anyway.