Add capability for different fuels within the same input emission inventory

[liammegill]

Currently, the input emission inventory only supports a single assumed fuel type (conventional kerosene). We should extend the emission inventories with one or more variables related to the fuel type used. Simple example would be an integer related to a fuel type, e.g. 0 = conventional kerosene, 1 = SAF, etc. Or, each input value also gets its own set of emission indices, with defaults in case they are not provided.

[liammegill]

Conclusion of discussion with @stefan-voelk today: We will include the section [aircraft] in the config file. Here, we define the variable types = ["KER", "SAF", "LH2"] as an example. The plan is that the input emission inventories will have an extra data variable, e.g. aircraft_type which corresponds to one of these strings. We can then define any further variables needed by OAC. The idea is that the user can define types as desired, the devs can add and remove input variables as required in development and the size of the input emission inventories is kept low.

So, for example:

[aircraft]
types = ["KER", "SAF", "LH2"]
KER.G_comp = 0.04  # comparative SAC slope G
KER.eff_fac = 1.0  # efficiency factor compared to kerosene
KER.PMrel = 1.0  # relative PM emissions compared to kerosene

If no type is defined, we will for now default to a conventional, kerosene-powered jet aircraft. This will be noted in the logs and output.