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
fixed array ops (checking types before using)
Fix the incompatibility with GPDB4.3 w/ ORCA

Pivotal Tracker: 57198180

LDA is not compatible with ORCA in GPDB 4.3. To avoid performance
regression, the only workaround now is to temporarily turn off ORCA
when executing LDA.
GPPKG: Change generated gppkg name

Naming conventions for GPPKG have been changed to enable easy detection of
open-source software version number. The change is required since 'gppkg'
performs installation/uninstallation using the gppkg name.
LDA: Turn off optimizer only when GPDB is 4.3

Pivotal Tracker: 58839874

Commit 14e2d74 added a fix for LDA by turning-off ORCA. The commit, however,
didn't add a check for GPDB 4.3 and hence led to issues with postgres and GPDB
4.1 and 4.2. This commit fixes that problem by checking if the GPDB version
is specifically 4.3 before turning the optimizer off.
LDA: Remove notice statements
Fix the order by problem in the sub-query and possible int32 overflow problem

    - Add ArrayType<int64_t> and MutableArrayType<int64_t> type trait
    - Rewrite the core query for training and prediction to avoid using internal order by
    - Change the type of model from INT4 to INT8
Robust Variance for Cox (Huber-White Sandwich)

Pivotal Tracker: 51528937

Other authors:
    Qian, Hai <hqian@gopivotal.com>
    Shengwen Yang <syang@gopivotal.com>
    Rahul Iyer <riyer@gopivotal.com>

Changes:
    - Design, and Implementation of Robust Variance for Cox
Clustered Variance for Cox (Clustered Sandwich)

Pivotal Tracker: 51529121

Other authors:
    Qian, Hai <hqian@gopivotal.com>

Changes:
    - Design and Implementation of Cluster Variance for Cox
set up about topic
Merge branch 'integration' of https://github.com/madlib-internal/madlib into integration