Add support for Lagrange multiplier in cfpdes

[Trophime]

I wonder if it's possible to simply add Lagrange multiplier as unknowns in cfpdes.
If not, It will be great to be able to do that

[vincentchabannes]

Currently, it's possible to have a lagrange multiplier on whole domain (as the pressure with Navier-Stokes), but not on marked faces. Implementation requires some work.