DateModel

[alecloudenback]

Contracts already use FinanceCore.Timepoint... which is simply Union{Date,<:Real). This PR on the main PR would:

• Define a DateModel which has two fields: a model (same model you would otherwise use in a Projection and valuation_date
• in various logic, use dispatch combining the underlying type within the contract and the model to determine the decimal time (yearfrac) relative to the val date in the DateModel
• Use BusinessDays, DayCounts, and logic from Miletus to determine dates of cashflows for real date bonds

Edit: the above won't work because each asset individually needs a day count convention. Make Timepoint a tuple of (Date,DayCount) instead?

Misc:

• Would FinanceCore.Timepoint better be defined as a SumType?