Lagrange Restrictions

[pjaap]

Seems to work for the two examples with periodic coupling

• Added BlockArrays
• Added Restrictons struct and a first CoupledDofRestriction

Problems:

• converting the block matrices to sparse matrices again is super slow BlockArray to Sparse quite slow JuliaArrays/BlockArrays.jl#78 (need to work around this)

[pjaap]

Also added MeanValueRestriction to restrict a scalar variable to a certain mean value over the domain. Tested in Example 252.

Still missing:

• doc strings
• CHANGELOG

[chmerdon]

Nice, but I think something is not working properly with the restriction_rhs. When I try to fix a nonzero pressure mean in Example252, it is still zero. When I stop after one iteration, the mean has the negative value of what I prescribed.