Periodic BCs

[cmhamel]

• Bookkeeping - need to read in e.g. nodesets/sidesets (others?) and make appropriate pairs

• Need new assembler type (or subtype that gets looped into the broader assemblers)

• Maybe the best way is to take the assembler from the dof system before constraints and take on constraints to the assembler? This may not make for clean splitting if that's desired in the solution approach

• Need testing

• Linear constraints to start