Getting started with bandicoot dev

@zoq @rcurtin

My plan is to get started with bandicoot development a bit. Once I get a better understanding of the codebase I will start implementing functions that are widely used in mlpack.

Implement zeros, a function to fill the mat with zeros. I went through the arma implementation.

I believe we have to add some files here:

• fn_zeros.hpp: which will have the main implementation

According to arma's implementation we need to implement another function called Gen() as zeros uses Gen

• Gen_bones.hpp
• Gen_meat.hpp
• Gen implementation for type Cube

I think I can start by defining Gen_bones.hpp, I will write the basic class definition first and push some code to make sure I am going in the right direction.

@conradsnicta any more points that I should keep in mind?

After some more discussion, we decided to implement join_cols, you can check this PR