BUG/ENH: Write STL Files from Triangle Geometry should have additional grouping options

[imikejackson]

Context

The user needs to read in a large number of STL Files using the "Combine STL Files" filter. That filter also creates an "Index" data array for each triangle and vertex as an indicator of which STL file it came from. Once all the STL files are into a single triangle geometry, we can manipulate that triangle geometry using Apply Transformation to Geometry filter. Then we want to write individual triangle files back out to the file system using "Write STL Files from Triangle Geometry". There is no way to leverage the "index" array that was created. 

Proposal

• Add another option to the filter to use the "index" array to separate the triangles back into individual STL files. There is currently the "Features + Phase" option but this requires more work on the part of the user, work that is non-obvious as they need to create a 2 component array to act as the face Labels. We could simply add a "Phases" option.

• Add a File Prefix option as a parameter to give the user more control over the naming of the output files

Discussion

Currently there is almost an ad-hoc sorting/hierarchical system with the Face Labels + Phases. Is there a way to allow the user to drop in any number of grouping arrays into a MultiArray Selection Parameter and then use those arrays to generate files using the index value from those arrays to generate the final output file name + the prefix that we will also allow them to set.