Automatically update eurusd.csv if required + introduction of unit tests

[quantx-heiko]

Hi @laroche,
I don't like the necessity to update the eurusd.csv file manually if it is outdated, so I adjusted the code to automatically download a current version if it is required depending on the transaction history data.
Additionally I redesigned the code to a more class based approached which allows to implement unit tests. Some functionality has been moved to a TastytradeHelper class. I started implementing a couple of unit tests for its methods.

From my point of view that make future code changes much more easier, if developers can check if unit tests are still running without errors. I'd love to see unit tests in the future which check that certain transaction history inputs deliver a certain tax data output.

Please review my code changes.

[quantx-heiko]

The unit tests can be executed either directly from VSCode "Test" tool section on the left hand side or directly from the Terminal using the following command:

python -m unittest discover -v -s ./tests -p "*_test.py"

[laroche]

Current code should already download the eurusd data if no local file exists, but normally I don't want
to depend on a working Internet connection and I often run in a loop to generate output tax data for all years.

I'll look at the code and check to merge it in.

best regards,
thanks for your new code,

Florian La Roche

[quantx-heiko]

It still will work without internet connection as long as the eurusd.csv file got the conversion rates for the required transaction dates available. I just didn't like the fact that the user needs to take special care of the conversion rate file if it is available but outdated.