Hi all!

I found the following issue with XDA restart. But first, let me start with a working scenario:

- let's have a simulation with 2 variables: 'u' and 'v', where 'u' lives on the whole domain and 'v' only on half of it.
- save the result into an XDA file
- when restarting, we recreate the EquationSystems, we add 'u' and 'v' sitting on the same domains
- we read the XDA file
- all is good and working.

Now if we get an XDA file (for example from someone else) - we may not know where each variable lived. The way to go would be to do:
- read the mesh file
- create EquationSystems object
- call read on the XDA file

However the XDA file does not contain enough info to properly add variables, see system_io.C:212 where add_variable is called with no subdomain param, which means all variables will live everywhere and thus the read from XDA file will fail. A solution for this problem would be to put additional info about the variable subdomain into the XDA file. However it should not break the old XDA files, so I was thinking to put there something like (1 2 3), which would mean a set of subdomain_id(). We could probably peek the stream for opening parenthesis to see if this info is available there or not and take a proper action based on that.

If you have any comments, please let me know. Meanwhile, I'll work on a patch that will enable this.

David Andrs