Add support for missing data in discrete-time models

[dimkab]

Modify DiscreteTimeSimulator::_simulate to support NaNs in some observation components.

• l410: adjust the condition to take partial NaNs into account (i.e. enter the block only if all components are NOT NaNs)
• l484, l490: adjust the obs for DiracObservations
• add utilities to factor multivariate Gaussians [optional, another PR?]

Prefer no dispatches :)

Tests: TBD

[DanWaxman]

Thanks Dima!! Here's a test I think should pass here.