Overwriting Restriction

[jeremylt]

For some nodal operators such as the BC operators in Ratel (and I think level transfer operators for multigrid?), it would be convenient to have a way to apply a CeedElemRestriction in CEED_TRANSPOSE that overwrites instead of summing into the target vector. Lots of ways to approach it, and I'm pondering on which I prefer. Feel free to comment

[jrwrigh]

I kinda lean towards an MPI-esque naming scheme. Maybe a CeedElemRestrictionSetReduceMode()? Where the mode can either be ADD or REPLACE?

[jeremylt]

ADD and INSERT would be my inclination, but yea, I like that approach