Define GRINS::invalid_var_index
Define GRINS::invalid_var_index

We'll use this immediately for initializing variable indices. Idea
is to be able to trip asserts if the variables don't get initialized
properly with libMesh. Right now, the default is not defined, but if
it's zero, this can cause problems if you haven't init'ed the variables
properly.

nicholasmalaya/grins,nicholasmalaya/grins,vikramvgarg/grins,cahaynes/grins,cahaynes/grins,nicholasmalaya/grins,vikramvgarg/grins,nicholasmalaya/grins,cahaynes/grins,cahaynes/grins,vikramvgarg/grins,vikramvgarg/grins,cahaynes/grins,vikramvgarg/grins,cahaynes/grins,vikramvgarg/grins