Utilities: Use madlib array_agg for GPDB < 4.1 and PG < 9.0

Updated utilities function is_col_exists to use the
madlib_schema.array_agg when the version of GPDB is 4.0 and version
of PG is 8.4. For this change, the function takes an extra parameter,
the madlib_schema name. This change was reflected in logistic
regression and cross validation where is_col_exists was being used.
LDA: Handle arrays passed as strings for earlier versions of GPDB/PG

GPDB < 4.1 and PG < 9.0 pass arrays as strings. These should be
tackled using mad_vec function provided in utilities.
LDA: Have mad_vec return a float array