Misleading warning message for assets decommissioned before the start of the simulation

[tsmbland]

When this happens we give a warning message in the form

Asset 'wind_onshore' with commission year 1993 and lifetime 25 was decommissioned before the start of the simulation

where lifetime is taken from the value specified in process_parameters.csv. The problem is that when max_decommission_year is specified for the asset, the lifetime of the asset is not actually represented by lifetime. E.g. in this case if max_decommission_year is set to 2020, then the lifetime (or more accurately the maximum lifetime) is actually 27 years so the warning message is incorrect.

This is important because the current message might imply that the solution is to increase lifetime, but actually max_decommission_year takes precendent so this is what needs to be changed to prevent the asset being decommissioned before the simulation.