Fix investor transaction amount calculations

[sophialittlejohn]

When querying investorTransactions all of the transactions that were considered tokenTypes were returning numbers far too small to be realistic (usually zero). By moving all to the currencyTypes the amounts were calculated properly again. Seems to me that the events actually always return the tokenAmount and therefore it must always be converted into the currencyAmount

TODO:

• fix tests

[hieronx]

Closing in favour of https://github.com/centrifuge/pools-subql/pull/61