Add option to export exceedance probability timeseries in iamc compatible format

[phackstock]

The motivation

In the AR6 workflow we had exceedance probability as a full timeseries and not only as a meta indicator. Adding this to the output is a requirement to reproduce the AR6 results.

The proposed solution

In src/gcages/post_processing.py in the PostProcessingResult class, add a method (or property) timeseries_exceedance_probabilities_iamc.
The code would be as follows:

...

def timeseries_exceedance_probabilities_iamc(self, format_name: str):
    exceedance_probabilities = self.timeseries_exceedance_probabilities
    exceedance_probabilities = exceedance_probabilities.reset_index()
    exceedance_probabilities["variable"] = (
        format_name
        + exceedance_probabilities["variable"]
        + " "
        + exceedance_probabilities["threshold"].astype(str)
        + "C|"
        + exceedance_probabilities["climate_model"]
    )

    exceedance_probabilities = exceedance_probabilities.drop(
        columns=["climate_model", "threshold", "threshold_unit"]
    )
    exceedance_probabilities = exceedance_probabilities.set_index(
        ["model", "region", "scenario", "unit", "variable"]
    )
return exceedance_probabilities