 need to read the matrix <code>X</code>
using <code>read.big.matrix</code> from <code>bigmemory</code> package.
The <code>prepare_data</code> function has a parameter <code>maxp</code>
which represents the maximum size (that is the number of elements) of
one part. If <code>X</code> is bigger, it will be split. It will be done
even if your matrix is big but you have enough RAM to read it in a
normal way. It may seem unnecessary, but it is worth to do because R is
not very efficient in dealing with big matric