Add ExodusII_IO public accessor for "var table"

[jwpeterson]

Currently we call exII::ex_get_var_tab() from ExodusII_IO_Helper::read_elemental_var_values(), but we only use the table locally within that function... it isn't saved in the class. This information could be useful in some situations, though, for example if you are trying to set up a System with subdomain-restricted variables that matches the way those variables are currently restricted in the Exodus file.

One approach would be to make the var table a public member of the helper class and then provide an ExodusII_IO API that returns it as e.g. a map<string, set<subodmain_id>> from variable names to sets of active subdomain ids for ease of use.