PCBDDC: special nodes should be selected as primal vertices even if they are shared by two subdomains only

they thus need to be communicated to relevant neighbours when performing graph setup
PCBDDC: reverted selection of primal vertices as before 1e1f2d9

PCBDDCGraphGetCandidateIS now returns index sets only if the user asks for them
PCBDDC: candidates should be always computed.

they are returned back only if the pointer is not null
PCBDDC: forgot to commit relevant changes to bddcprivate.c and bddcprivate.h
PCBDDC: index sets should be set to null before computing them since thay have to be returned
PCBDDC: a positive tolerance breaks the code with BAIJ formats.

Now PCBDDC infers symmetricity only if the user set it
PCBDDC: using MatIsSymmetricKnown for symmetry checking will never break codes
PetscDualSpace: Added a Simple space
- Its just a collection of functionals
PetscFV: Now can hold a quadrature object
- This allows projection onto the piecewise-constant space
PetscFV+Doc: Fixed comments
DM: Fixed memory leak
Plex: Fix DMCreateDefaultSection_Plex() to handle FV discretizations
- I will check CLASSID until a superclass for FE/FV becomes clear
Plex+Doc: Removed useless comment
Plex+FE+FV: Fixed projection and L2 diff for FV
- Make a fake, simple PetscDualSpace for FV when projecting
- Added PetscFE/FVInterpolate_Static() for synthesis of field from coefficients
PetscDS: Added Riemann solvers and contexts for each field
PetscDS+Doc: Added function docs and made some static
Plex+TS ex11: Now default section computation can handle an FVM discretization
DMTS: Reorganized DMTS for Plex
- Now FVM computation functions look just like FEM
- Riemann solvers are specified to DS, just like f0/f1
- Removed explicit DMPlexTS structure, now just attach to DMTS
- Changed DMPlexTSSetRHSFunctionLocal()
- Added DMPlexTSComputeRHSFunctionFVM()
- Added minradius to DMTS
Plex+Ex: Label completion is now done automatically
TS ex11: Now specify RHS function directly
- Riemann solvers now return void so that they are eligible for OpenCL