pio reader fixes and changes (paraview/release)
Fixed a bug where sometimes a variable was determined to be a string when it was actually numerical data.
Modified parsing of the .pio file. Comments can now be delimited with '!' as well as '#'. Comments can be placed in the middle of a line, and possibly on the same line as a command. Spaces are allowed at the beginning of a line, as well as multiple spaces are allowed between arguments.
Changed error messages to be clearer about what is wrong.
This is the same set of changes as !7644 (merged), except this MR is targeted toward the paraview/release branch.