MrkvNow into model state shocks['Mrkv'] in AggShockMarkov and KrusellSmith models

[sbenthall]

Fixes #933.

In the AggShockMarkov and KrusellSmith agents and associated economies, the MrkvNow property was being set as an object attribute, despite being a model variable. It's an aggregate property in both cases.

This PR replaces these with a shock Mrkv. This is small progress towards #920, which otherwise can be done with simple search-and-replace operations.

Mrkv is a shock and not a state in this implementation because the way Market interactions with the Agent currently works, if the Market sets and agent state in its sow step, it will be rolled over into the state_prev namespace during the simulation in the cultivate step. Because here the Mrkv state really is exogenous to the agent, shocks seems more appropriate, though I hope we can scaffold this a bit better in, say, v1.1

• Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[sbenthall]

Needs CHANGELOG. Merge after.