Change the currency conversions

[Femkemilene]

Change the currency conversions in the code. We should be able to code this up using the histend of BCET, rather than hardcode as currently.

[Femkemilene]

As everything should now be in the same currency units, I plan to completely remove this. If we need to add this back, we should add it back as a function in support, with appropriate documentation, as it is insufficiently clear how this works.

It needs removing in main, RLDC (and in lcoe for the carbon tax in the cascading branch).

[Femkemilene]

We don't necessarily need to align with E3ME here. Those conversions are always a bit iffy. If we instead just use https://pypi.org/project/CurrencyConverter/, we also don't need to update the EXX file etc all the time. Any opinions on deviating here?

[Femkemilene]

The CurrencyConverter module is exceedingly slow (half a second per call), so we might need another solution. Perhaps that is a one-off call such that the currency-conversion is saved dynamically upon the first run after a download, and then kept afterwards.