Add CeedElemRestrictionCreateUnsignedCopy

[jeremylt]

I need to double check the design, but I think this should do what we need.

[jeremylt]

Note to self: I may have a leek on this branch. Need to investigate.

[jeremylt]

Ok, "leek" found and fixed

[sebastiangrimberg]

I like this idea and wonder if it can be used to simplify #1196 by removing the CeedElemRestrictionApplyUnsigned interface function added there, having the user instead create a copy (CeedElemRestrictionCreateUnsignedCopy) and then just use CeedElemRestrictionApply. This would remove the additions to the API which are now unnecessary.

[jeremylt]

I'd be inclined to move the function to the ceed/backend.h interface if we don't intend it to be used in general anymore.

[jeremylt]

Ok, moved. Really, I should cook up a test for CeedElemRestrictionApplyUnsigned so that there is an easy point of debugging when the GPU backends start adding signed restrictions.