Missing `PCFieldSplit` support

[oriolcg]

The current implementation does not allow to use pc_fieldsplit preconditioners of non-strided block matrices, i.e. ConsecutiveMultiFieldStyle. I believe that to enable this feature we should have an implementation of the function PCFieldSplitSetIS and the associated index set struc IS.

Am I missing something? If not, how should we proceed?

[carlodev]

I have implemented the index set IS in my fork.
You can take a look also at the simple test., it also works on multi procs. .
Now I am trying to use it for the Stokes case
I suppose the big problem will be in parallel, how to split the indexes over the procs properly