Require precise date for setting up a policy environment

[hmgaudecker]

Frequently, the policy environment changes throughout the year. Currently, we can call it with an integer for a year, a month, or a precise date. The former two may create ambiguity (we probably default to 1 Jan if a year is provided, but I would actually have to look that up).

Just as for data, we should require precision from the user instead of making assumptions.

• Change parser
• Check docs
• Fix tutorials