Allow more current lighting usages

[DaJansenGit]

With the change from xml -> json for use conditions the previous existing parameter maintained_illuminance [Lumen/m²] was removed. The parameter describes the needed illuminance for the different usages of zones (use condition) and is available in DIN 18599-10.

Currently the lighting power is inserted as a fixed value which seems to come from SIA 2024, didn't found the value in DIN 18599-10. Anyway this fixed value doesn't allow to calculate the specific power of lighting based on different types of lighting. (see here)

To allow more current lighting setups and more flexibility I propose the following:

• Add the maintained_illuminance again to have the needed illuminance for each zone
• add the additional paramter lighting_efficiency_lumen which is the german "Lichtausbeute" from above. This parmeter has the unit [Lumen/W_el]
• This way we can calculate the lighting_power by
  lighting_power = maintained_illuminance / lighting_efficiency_lumen [W/m2]

the lighting_efficiency_lumen should be an additional parameter for use_conditions.py with a default value which can be adapted based on the technogies from the linked image above. We could provide some examples in the docstring like "traditional light bulb: 20 lm/m2, LED: ~ 150 lm/m2" etc.

[DaJansenGit]

@PRemmen Was there a reason to remove the maintained_illuminance when you switched from xml -> json?

[PRemmen]

@DaJansenGit no reason that I can remember

[DaJansenGit]

@DaJansenGit no reason that I can remember

Ok, thanks :) Then I will proceed with the proposed changes and do a PR.