A handful of bugs quickly cropped up in 3.31.0 that justify this .1
release.  While there are always more bugs to be fixed, hopefully these
fixes help some people out.

There are a few new features in this release, but they're really small.
In a normal (larger) .0 release, these features probably wouldn't even
get mentioned here.

The real story of this release is the bug fixes - and it is a story mostly
about structures.

As it turns out, CalculiX B32R's are better than B32's and per-element
normal vectors don't work (they are still the default).  An option to
disable beam element per-element normals has been added -- it should be
used until CalculiX fixes their bug.

Features:
 - Add ResetMeshDisplay button to StructScreen
 - Structural assemblies can now be inserted with rest of vsp3 file.
 - Measure probes and rulers can now be inserted with rest of vsp3 file.
 - Drop Ubuntu 18.04 from automated build system.

VSPAERO Updates and Fixes:
 - Adding sensitivities for reference quantities.
 - Improve setting CpSteadyMin/Max in steady flow.
 - Bump and synchronize versions to 6.4.5.

Bug Fixes:
 - Improve Calculix handling of beam elements.
 - Fix crash when writing *.vspgeom file from imported mesh.
 - Fix Vinf activation with stability choice.  Thanks Daniel R.
 - Fix tags for CompGeom half-mesh.  Fixes *.vspgeom half-mesh panel mode.
 - Fix consistency of StructScreen current structure choice.
 - Fix StructAssemblyScreen height for various recent changes.
 - Fix writing of ghost nodes for half-geom *.vspgeom files.
 - Fix ID handling for copy/paste and insert of structures. Thanks Bryan.
 - Fix subsurface tagging bug with quad meshes.