Problems with scalability on size of design-matrix

[Blunde1]

agtboost does not scale well with large n and m.
In principle, it should scale as well as e.g. xgboost with algorithm set to exact.
There also seems to be problems with memory-consumption for large datasets.

Goal
Make agtboost scale well in n and m.

Suspission
I suspect this is due to the re-declarations/initializations of u_store and u vectors and cir calculations in split_information().

• Find a smarter way to do these calculations. See if this fixes scalability issue.