Switch to pure PETSc for ACV. There appears to be a saving in memory, and no additional cost to assembly. Note that the insertions are suboptimal, but this is a fuction of the inefficiency of the chosen block structure, which could possibly be improved.
Merge branch 'fluidity-core/MultiFluids_Dev' of github.com:jrper/MultiFluidFluidity into fluidity-core/MultiFluids_Dev