Weird behavior in multiphase mfd results.

[ryar9534]

Describe the bug
Something seems off in flow results obtained using the multiphase mfd implementation (beiraoDaVeigaLipnikovManzini inner product). I ran a flow only staircase example (provided here) and you can see the difference in the CO2 plumes obtained with TPFA (first) and MFD (second). This is obtained with an up to date branch.

Some things to check:

• does the difference go away with gravity? (I think it might)
• Is it because of source flux injects into multiple columns of cells?
• Right now, I initialize to a constant pressure, even when gravity is on. If the initialization step is included, I think there are also differences there.
  testCase.zip