graham-dds 7a0dc4d264 Add properties to portfolio model
1. Funding duration

Returns a tuple of the earliest period of performance start date and
latest period of performance end date for all active task order in a  portfolio.

2. Days to funding expiration
Returns the numbei of days between today and the lastest
period performance end date of all active task orders

3. Active task orders
Returns a list of a portfolio's active task orders a
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