LoadVTK will now be elected when using the generic loader

LoadVTK will do 3 simple checks to determine how confident it is:
if the file isn't ASCII and ends in .vtk it's very confident (80)
if it ends in .vtk it's fairly confident (60)
if it's not ASCII it'll attempt it only if all other non-ascii loaders reject it (15)
otherwise it'll reject it (0).

LoadVTKTest will now check to see if the generic loader returns a not-null object

Refs #7420