ETF: enter preliminary lump sums piece price

[OlliL]

I know of two different ways how preliminary lump sums are communicated by the bank to the customer

1. an amount for each month is communicated. This is currently implemented
2. a price per piece is communicated and the amount for each month has to be calculated based on this one price. This is not yet implemented but adding this additional method is the goal of this issue

• validate that for the monthly way the piece price is null and for the piece mode, all the monthly amounts are null when updating or creating
• verify that with tests

[OlliL]

Server part should be done.
Now its about making it possible to enter and display data in the new way (and old way) in the client.