[MRG+1] #3364 warm start in random forests

Related to issue #3364.
The RF model will convert internally to 32 bit float, so this is the same in this case and should be negligible on such a small dataset anyway.

It's only interesting to load the data directly in 32bit format if you have it stored on a hard-drive for instance to avoid the memory copy. In practice this is only important for dataset that are big enough to use a large fraction of the RAM.