Functionality to merge multiple forests into a single forest

[andrewherren]

Suppose we have two forests, each with k trees. We would like to be able to combine these two forests into a single forest with 20 trees. To make this happen we need the following at the C++ level

• Append / concatenate functionality which modifies a forest to include another forest's trees
• Ability to add a constant value to all of a tree's leaves
• Ability to multiply all of a tree's leaves by a constant value

At the R level

• Wrap the append / concatenate functionality in a method of the ForestSamples object
• Wrap forest arithmetic operations in a method of the of the Forest and ForestSamples objects

At the Python level

• Wrap the append / concatenate functionality in a method of the ForestContainer object
• Wrap forest arithmetic operations in a method of the of the Forest and ForestContainer objects

Longer-term, it would be nice to give users an expressive API that allows for conceptually simple operations like "turn this container of 20 forests of 10 trees each into a container of 10 forests with 20 trees each," but for now this will be "low-level" functionality