- moved reset methods to this class
- Add method `recalculateForGroupAndCurrency` that accepts a $limitCurrency as third argument.
- Add `recalculateAccountsForCurrency` and `calculateTransactionsForCurrency`
`recalculateForGroupAndCurrency` can recalculate any foreign amount back to the primary currency (just like the class was already capable of) BUT limits the action to foreign amounts in $limitCurrency. This greatly reduces the rework necessary. This means the method can be safely called when changing currency exchange rates.
Not all methods in `recalculateForGroupAndCurrency` actually care about the given $limitCurrency but the methods that don't aren't doing much anyway, so it's OK to recalculate everything even though its not necessary.
