* make DMatrix._init_from_npy2d only copy data when necessary
When creating DMatrix from a 2d ndarray, it can unnecessarily copy the input data. This can be problematic when the data is already very large--running out of memory. The copy is temporary (going out of scope at the end of this function) but it still adds to peak memory usage.
``numpy.array`` copies its input no matter what by default. By adding ``copy=False``, it will only do so when necessary. Since XGDMatrixCreateFromMat is readonly on the input buffer, this copy is not needed.
Also added comments explaining when a copy can happen (if data ordering/layout is wrong or if type is not 32-bit float).
* remove whitespace
9b2e41340b