[cass_fiscal] Dividing and cleaning up the two cass_fiscal lectures (#485)

* Tom's July 11 edits of a lecture and splitting lectures

* Tom's troublesome attempt to split cass fiscal lecture into two parts, July 12

* updates

* Tom's July 13 edits of the cass fiscal policy lectures

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Co-authored-by: thomassargent30 <ts43@nyu.edu>

Author: HumphreyYang

lectures/_toc.yml

@@ -74,6 +74,7 @@ parts:
   - file: cass_koopmans_1
   - file: cass_koopmans_2
   - file: cass_fiscal
+  - file: cass_fiscal_2
   - file: ak2
   - file: cake_eating_problem
   - file: cake_eating_numerical