Add number of modules field to solar panel preset

[kallejre]

Suggesting to add support for key generator:solar:modules to solar panel preset. This could be simple numerical entry field. It's relatively simple tag to map from aerial imagery. Definitely easier to verify from aerial than power output wattage, but i personally find it rather difficult to memorize how was that key called. Not really sure what else to write here. Due to usage of alternative energy steadily growing and being incentivized in many countries, usage of this tag has been skyrocketing for past year.

Another idea i wanted to suggest was drop-down field for location-tag, but reading possibly related #101 i think it might not be good idea any more.

[kallejre]

New year, new imagery, new solar panels and issue rises again. Apparently i have already created this issue. Took a deeper look at this repo's structure and looks like that fields key in photovoltaic.json should contain additional line "generator/solar/modules". Then new folder solar should be created into generator directory and into that file modules.json with following content:

{
    "key": "generator:solar:modules",
    "type": "number",
    "minValue": 0,
    "label": "Solar panel modules",
    "placeholder": "8, 10, 16..."
}

I'm not really familiar to making those changes myself due to amount of overhead work needed to set up environment and even if i did, i lack skills to try it out and verify it works before submitting PR. This information above is probably missing something required for adding text labels.

[MMK21Hub]

Perhaps a more descriptive label would be "Number of modules" or simply "Number of panels". Including the phrase "Solar panel" in the label feels redundant to me, as I know that I'm editing the attributes of a solar panel.