move MarkovArray exact_match and T_min sampling to the Distribution class

[sbenthall]

Some of the current model implementations using a Markov array do not use straightforward probabilistic sampling for transitions.

• CobbDouglasMarkovEconomy restricts MarkovHistory output to be valid for calcDynamics: https://github.com/econ-ark/HARK/blob/master/HARK/ConsumptionSaving/ConsAggShockModel.py#L2500-L2598
• In KrusellSmith, the EmpNow value is sampled with something more like an "exact match" of final employed/unemployed numbers: https://github.com/econ-ark/HARK/blob/master/HARK/ConsumptionSaving/ConsAggShockModel.py#L965-L980

These alternative modes of sampling should be moved to the Markov class in Distribution.