Support DC-connected batteries with multiple MPPT inputs

[cpaulgilman]

For a PV-Battery configuration with DC-connected batteries and more than one string and multiple MPPT inputs, simulations fail with the following message:

exec fail(pvsamv1): DC-connected batteries do not work with multiple MPPT input inverters.
Simulation pvsamv1 failed :exec fail(pvsamv1): DC-connected batteries do not work with multiple MPPT input inverters.
monthly_grid_to_batt not assigned
Simulation cashloan failed :monthly_grid_to_batt not assigned

Note that for systems with one inverter, the SAM UI will not allow multiple subarrays with one MPPT input. That prevents a possible workaround to model multiple MPPT inverters as single-input inverters where SAM averages the operating voltages of the strings. It is possible to model multiple subarrays with single-MPPT inverters when there is more than one inverter.

File for testing: pv-battery-multiple-mppt.zip

• Simulation error: "multiple MPPT" case
• UI error when switching to System Design page: "one single MPPT inverter" case
• No error: "three single MPPT inverters" case