Add Rounding Convention

[nbundi]

In general, different conventions re the rounding of fractions of prices, interest accrued, etc are used on different markets, by different entities, etc.

In order to define unambiguously the exact payment obligations, price, etc the rounding convention (aka decimal point convention) needs to be added on the contract-level.

Most common conventions:

• nth decimal
• n/m fraction

Question: do we need rounding up, down qualifier?