draft: adds support for symbol change

[avion23]

This is my proposal for symbol changes.

Test data attached.

symbol-change.csv

It would be nice to also check if the replacement symbol is the correct one, but I think that's not possible with the current structure.

Does this work? Is there a better approach?

[avion23]

symbol-change-mock.csv
I've tested with symbol-change-mock.csv. That's the same data, but the symbol change was removed.
This results in

Stillhalter-Gewinne                     109.85  109.85
Stillhalter-Verluste                    -67.43  -67.43
Stillhalter Gesamt                       42.42   42.42

while the original csv file shows

Stillhalter-Gewinne                     222.43  222.43
Stillhalter-Verluste                   -180.01 -180.01
Stillhalter Gesamt                       42.42   42.42

This implies the trades are recorded twice during the symbol change which is wrong.