Improve UMFPack vector chooser

UMFPack solver can handle nested matrices due to the use of flat-vector/matrix algorithms, however, the UMFPackVectorChooser does not reflect that (e.g., a range type of a BCRSMatrix<BCRSMatrix<...>> is a BlockVector<BCRSMatrix<...>>). This MR, simple makes the UMFPackVectorChooser more friendly to those nested types.

• Solve nested type BCRSMatrix and FieldVector with recursion of UMFPackVectorChooser

• Also, resolve the UMFPackCreator type with SFINAE on the UMFPackVectorChooser instead of arbitrary rules of the creator (the vector chooser already knows the rules, there is no need to repeat them)

• Allow custom vectors in the UMFPack solver

• Add README