Limit train samples (#2809)

* Limit training samples size to 2GB

* simplified DISPLAYLEVEL() macro to use global vqriable instead of local.

* refactored training samples loading

* fixed compiler warning

* addressed comments from the pull request

* addressed @terrelln comments

* missed some fixes

* fixed type mismatch

* Fixed bug passing estimated number of samples rather insted of the loaded number of samples.
Changed unit conversion not to use bit-shifts.

* fixed a declaration after code

* fixed type conversion compile errors

* fixed more type castting

* fixed more type mismatching

* changed sizes type to size_t

* move type casting

* more type cast fixes