Add liquidity constraint to perfect foresight model examples

[alanlujan91]

Please ensure your pull request adheres to the following guidelines:

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

The notebooks mentioned in econ-ark/DemARK#35 were moved from DEMARK to HARK/examples.

This PR updates model notebooks to account for the existence of BoroCnstArt in the perfect foresight model. It should only be merged after this feature has been included in a public release of HARK, otherwise the notebook will break; likewise, it should be merged immediately otherwise the notebook will break.

The changes are migrated on this PR.

Additional changes made:

• time.clock has been deprecated since python 3.3 and removed in 3.8; also it's not used anywhere in the code, so i have removed it
• I've changed hist_x to history["x"] where needed
• changed unpackcFunc() to unpack("cFunc") where needed