Fixed default value for BoostedRegressionTree.Alpha.

Actual values will be allowed in future versions of VisTrails, but
currently, you're limited to the serialized value as a string.
Use a hash -> dirname map to better support jobs run in loops.

Creates a vt_hashmap.dat file in the session directory that is used to
link the signature (computed in Module.compute) with the directory
name (e.g. "brt_1").