Add a tag set to subdomains

This allows us to add and remove tags from a subdomain.  We can also
check to see if a subdomain has a certain tag.
Use bfam_locidx_t for the number of subdomains
Add ability to match subdomains given tags
Add library tags to subdomains
Use NULL terminated tag array to match subdomains
Add runtime endian check
Remove spaces from dg quad subdomain name

We remove the spaces from the subdomain name to make it friendlier to
use as part of a filename.
Add vtk_write_file function to subdomains
Add vtk write file function

This adds a function that loops through the subdomains and tells them
to output vtk files.
timestep start