ENH: Added further Solve methods to VNLSparseLUSolverTraits

Three new methods have been added to allow the user to reuse the internal solver.
One use case for these new methods would be for problems where the matrix can be
decomposed once and reused multiple times (i.e. multiple back-substitutions)

Change-Id: Idd392dc545bfc1c1f0fbda95908b29cf19284127
Merge topic 'VNLSparseLUSolverReuseDecomposition'

190c4599 ENH: Added further Solve methods to VNLSparseLUSolverTraits