Add ESDL Model Author's intended timeframe

[jpwijbenga]

ESDL is used for simulations. May include Profile IDs referring to existing profiles of limited time. They may include commissioning/decommissioning dates, that give an idea about which timeframes this ESDL model may be simulated.

But missing is info on what this ESDL model's intended and usable period is. Eg. This model can be used to simulate 2030-01-01T00:00Z until 2050-01-01T00:00Z (so with the last timestep being from 23:00 to 00:00). Model authors can indicate this way that they guarantee the profiles and asset information is correct for this timeframe, and this info can be used to check before doing a run.

[edwinmat]

This is exactly what the date attribute of the esdl.Instance class is meant for. It allows specifying a specific date or a timeframe for which the instance of the energy system description is valid. See https://energytransition.github.io/#router/doc-content/687474703a2f2f7777772e746e6f2e6e6c2f6573646c/AbstractInstanceDate.html