Better documentation on setting temporal resolution

[fhg-isi]

a) Please include hint at

https://pypsa-eur.readthedocs.io/en/latest/configuration.html#snapshots

on how to set temporal resolution. I think it can be done using

sector_opts:
  - '12h'

https://pypsa-eur.readthedocs.io/en/latest/wildcards.html#the-sector-opts-wildcard

but I am not sure about it.

There is also

 clustering:
     temporal:
        resolution_elec: false
        resolution_sector: false 

https://pypsa-eur.readthedocs.io/en/latest/configuration.html#clustering

b) However, similar to snapshots wildcard, I would expect a global parameter to set temporal resolution,
that is then applied at all relevant places. (Or a "step" parameter for the snapshots).

c) Maybe it would make sense to include an extra article "Temporal resultion" next to "Spatial resolution":
https://pypsa-eur.readthedocs.io/en/latest/spatial_resolution.html
to clarify how PyPSA considers time steps.

That way, when searching for "temporal resultion" in the help, the results would be less distributed and confusing.

Related: #1165

[koen-vg]

I guess when you are saying "a global parameter to set temporal resolution", you mean that there should be a single configuration option for temporal resolution for both electricity-only and sector-coupled networks. See also my comments in #1277. In general I agree that there shouldn't be any reasons for having different temporal resolutions for elec- and sector-coupled networks within the same configuration.