optimize vtk file read-in behavior

[fractalsbyx]

Previously each vtk input would be read/parsed multiple times if it contained multiple needed fields. Now it will read each vtk only once. (per processor)

[landinjm]

Just to clarify, this rearranges the loop structure of the vtk read in so the file is parsed once for all input fields rather than once for each field ( O(n) to O(1)?

Otherwise, there are just a few small fixes.

Thanks for the PR!