RF: Use NULL::integer[] when no continuous features

[iyerr3]

JIRA: MADLIB-1219

When variable importance is enabled, to compute importance score,
distribution of the categorical and continuous features are computed.
For continuous features, this function requires initializing a vector of
length = number of continuous features. When there are no continuous
features, this initialization fails.

This commit fixes the issue by inputing a NULL::integer[] vector when
there no continuous features.

Closes #249

[asfgit]

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[fmcquillan99]

See https://issues.apache.org/jira/browse/MADLIB-1219 for results from my tests.

LGTM

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[kaknikhil]

The changes look good. +1 for adding an install check test.

I noticed that con_features is populated by decision_tree.py:_classify_features which does something like con_types = ['real', 'float8', 'double precision', 'numeric']. Is it better to call the utilities function is_psql_numeric_type ?

[iyerr3]

@kaknikhil Thanks for the suggestion. It makes sense to use that function with integer types excluded (since DT does not treat integer as continuous). I'll add that as a separate commit since it's unrelated to this PR.