Add helpers for calculating salary

[sohkai]

Transform salary per second to salary per year or month, based on the amount and the token's decimals.

In particular, we'll have to be careful and assume that some "tokens" may be fake.

[JovianBrowne]

@pythonpete32 or @nivida we need a little more context. Are these "salaries" inside of the DAO? What kind of/ Whose salaries are these?

[nivida]

Thanks for tagging me here. I will move this issue now to the Conception state in Linear and as soon as those things are clarified can we again plan it in. I thought this issue is more straightforward :-)