Specify bounds on multivariate distributions

[mnwhite]

The limit attribute of DiscreteDistribution instances links to the underlying continuous distribution from which it was generated, and also has information about the discretization parameters. There's still some work to be done on standardizing the limit dictionary, but its functionality has now been extended to include infimum and supremum entries, and for these to be handled correctly by combine_independent_discrete_dstns. The information in these entries (the bounds on each dimension of the distribution) is now used by ConsIndShock's solver to find the "worst income shock realization".

As described in the commit notes, this doesn't actually affect the consumption function solution very much. The user needs to both add aXtra points very close to zero and include a tiny point mass at the infimum for the consumption function to actually have gridpoints on the "almost constrained" portion.

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