Read the number of global variables present in the Exodus file when
we read the header, and don't try to initialize the global variables
if there are already some global variables present.
Revert "Adding _timestep_offset to hopefully avoid overwriting data when appending."

This reverts commit f7a552fb0c97eeba57a01fac1edbbb62a71e33d2.

We eventually decided that, since we sometimes *do* actually want to
overwrite prior solutions when appending, we must leave the timestep
to be written completely under the user's control.  This means more
flexibility, but more potential for accidental overwriting.