PCBDDC; add flag to invert or not Stildas

small fixes

PCBDDC: multilevel fixes

PCBDDC: solved warning from gcc for uninitialized variables

MATIS: move preallocation during MatISGetMPIXAIJ into a separate routine

Schur subsets can take advantage

PCBDDC; bugfix

PCBDDC: added initial implementation for adaptive selection of constraints

positive definite case covered only
untested for complex

generalized eigenvalue problem is inv(sum(inv(S_F))) = lambda inv

PCBDDC: added initial implementation for adaptive selection of constraints

positive definite case covered only
untested for complex

generalized eigenvalue problem is inv(sum(inv(S_F))) = lambda inv(sum(inv(St_F)))
this is the right one for 2D problems and for faces in 3D; it is an heuristic for edges in 3D

tested with H(div) and H(curl) (3D) and works like a charm

cholesky support for matrix inversion should still be added

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PCBDDC: add slot for vertices in sub_schurs data structure.

This will also be helpful for adaptive selection

PCBDDC; added slot for threshold when adaptively computing constraints

PCBDDC; clean up of sub_schurs/deluxe code

* options database are now part of sub_schurs setup
* eliminated unneeded field from sub_schurs structure
* added 2 interface functions to init and setup s

PCBDDC; clean up of sub_schurs/deluxe code

* options database are now part of sub_schurs setup
* eliminated unneeded field from sub_schurs structure
* added 2 interface functions to init and setup sub_schurs when setupping the preconditioner
* if a change of basis is requested, releveant matrices are projected
* still need to be tested when the BDDC is reused

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PCBDDC: compute sum of Stildas

PCBDDC; added Stildas to sub_schurs data structure

still don't know if they should belong to the structure or just their sums

PCBDDC: added initial support for computation of Stildas

the computation is very intensive, need to find an alternative

PCBDDC: added private function to explicitly compute local schur complements

PCBDDC: removed old SubSchursSetUp code

PCBDDC: added support for explicit computation of Schur complements using MUMPS in sub_schurs code

PCBDDC: store principal minors of local Schur complements by columns

This shoudl facilitate the computation of adaptive constraints

PCBDDC: remove unneeeded index sets

PCBDDC: small fixes

PCBDDC: move computation of explicit schur complements from deluxe code to sub_schurs setup

PCBDDC: make deluxe->seq_mat and sum of schur complements members of sub_schurs

PCBDDC: added a slot for local neumann matrix in sub_schurs (for future use of MUMPS)

solved a bug with external packages and 0 sized seq_ksp

PCBDDC: added a second sub_schurs structure to pcbddc data structure

Begin reorganization of SubSchurs code for code reuse for adaptive selection of constraints

PCBDDC: make sub_schurs a member of PC_BDDC data structure

PCBDDC: rewrite sub schurs setup

in the cheap version, interior problems are all the same.

Merge branch 'master' into mlange/refactor-plex-distribute

Conflicts:
src/dm/impls/plex/examples/tests/output/ex12_1.out
src/dm/impls/plex/examples/tests/output/ex12_2.out