Bcast xtrue to non-root ranks

[rayegun]

I believe this is correct, I haven't run it though. This should fix the issue we faced with xtrue being different when the number of ranks are > 1.

[xiaoyeli]

This is a good suggestion. Your patch has a small error: P0 (root) should also participate in MPI_Bcast().
Instead of merging your PR, I updated the macro files that generate all the pdrive_ABglobal.c, with 3 precisions and 4 different example drivers. They are pushed in master branch on github.

You can close this one now.