Issue #61: Update Cathod Pressure to be 30bar

[nRiccobo]

• added P_STD from scipy physical constants, which is 1 bar in Pascals
• mods to calc_open_circuit_voltage() include updated p_cathode, split up partial pressures for hydrogen/oxygen according to Dalton's law.

Issue #63: Nernst equation was incorrect. This issue only became became apparent when adjusting cathode pressure.

• Updated test_cell.py, test_stack.py, test_run_electrolyzer.py, test_run_lcoh.py to account for the new model change.

Yes. All tests have been updated and pass. However, we really should look at splitting up some of the tests like run_electrolyzer and run_lcoh that are so dependent on small changes to the model.

Feature or improvement description Addressed the bug #63 issue. Fixed the partial pressure calculations within the cell.py script.

Impacted areas of the software Calculations to LCOH and hydrogen output will be slightly different due to the different cell voltage and thus current density.

[nRiccobo]

Strange. It failed the test_run_lcoh.py, but I was certain that I added and committed the updated values to make it pass the test.

[nRiccobo]

There was a change that I committed, but did not push to my fork prior to the pull request. Now it should have passed everything.