Fixed signed vs size_t comparison warning (yyleng)

All current available versions (from 2.5.33 to 2.5.37) of flex declare yyleng as int instead of size_t.
Documentation: Remove TOC links to non-existent sections.

Updates in following modules:
    - Logistic regression
    - Cross validation
Clustered variance: Fix issues with incorrect casting

Pivotal Tracker: 58728080

Changes:
    - Fix integer to boolean cast bug in clustered variance for logistic regr
    - Fix missing drop cast to allow multiple madlib schemas in DB
Linear/logistic: Add explanation for p-values being zero instead of epsilon

Pivotal Tracker: 58728080

Probablilty values computed on a finite-precision machine can sometimes to be
too low. In such cases, we usually threshold the value and print it as 'zero'.
For statisticians, this has a different meaning that saying '< epsilon'.
We acknowledge this by adding a note in the documentation.
Cox Prop Hazards: Add NULL Handling

Pivotal Tracker: 57195006

Changes:
    - Added NULL Handling to allow NULLS in independent and dependent variable
    - Added 'num rows processed' and 'num rows skipped' values in summary table
    - Ensured that zph residual skips over NULL value rows while computing the
    residual statistic