Summary: New module to produce summary stats for table

Pivotal: 42751193

'summary' is a generic function used to produce summary statistics
of any data table.  The function invokes particular 'methods' from
the MADlib library to provide an overview of the data.
Improve docs for elastic-net and cross-validation
Improve cross-validation doc
Added check for GPDB<4.2 to ensure array is not treated as string
Corrected a bug caused by the previous commit
Utilities: Perform string to array for GPDB 4.1

Changes:
    - String to array was being performed for versions less than
    GPDB 4.1. This has been changed to transform for versions less
    than GPDB 4.2.
    - Summary: ntile_array was transformed from string to array
    but the case where ntile_array is NULL was not handled earlier.