The C language doesn't really know about matrices or 2-dimensional arrays. It would arrange a matrix in memory
as a series of arrays of same length each representing a row. It would then convert the 2-dimensional indexes
as a single index in this array. This exercise uses the same trick to write functions manipulating such a "fake matrix".
The first step is to understand how we are going to allocate memory for such a fake matrix. Implementing
this function will ensure you've understood how things are supposed to be in memory.
