[Users] [Patch] allow direct calls to the Periodic thorn back-bone

Dear Einstein Toolkit users,

some time ago I wrote a patch to allow external thorns to perform direct calls to the low-level Periodic API, as I needed to apply periodic conditions only on selected grid variables.

I proposed this patch to Erik, who suggested to implement some improvements and to discuss it on the mailing list. After having implemented his suggestions, somehow I forgot to post the patch also here...

The way in which it works is by simply moving the "logic" (handling of errors and parameters) into Periodic_ApplyBC to make BndPeriodicVI "thorn agnostic".

Best, David