add Cashflows datastructure

[alecloudenback]

New branch due to changes to master, replaces #77

• Tests
• Other methods
  • duration
  • convexity
  • spread
  • others?
• How to handle/dispatch on vector/StructArray of cashflows
• may need to upstream to FinanceCore to support IRR
• Confirm little to none performance penalty

Followups:

• there are functions like IRR which will work on vectors of Cashflow but not on generators returning cashflows. This was simple enough to work around with, e.g. present_value but there are other cases like IRR which don't dispatch on the value of an iterable because they need to know the information of the entire iterable. Is there a way to generalize the dispatch?
• minor breaking change that I didn't realize was an option was to specify a vector of discount rates as the first positional argument. This should be removed Cashflows or not and point users to Yields.jl

[alecloudenback]

implemented in FinanceCore/FinanceModels