Add pc fieldsplit

[carlodev]

I have tried to extend the ability of the package to manage PCFieldSplit options in PETSc for MultiFields. I have added the new ´IS´ PETSc type and several C functions for using the new type. Looking at the code, I collect the global ID of the equations/dof for each processor of each field. A new struct PETScFieldSplit has been created. The user can easily specify the fields and a tag name to identify the fields, such as:

FS = PETScFieldSplit([U,P], ["vel","pres"]; show_idx = true)
solver = PETScNonlinearSolver(mysnessetup, FS)

any constructive comments to improve/expand functionality are very welcome