Build a matrix with a function : makemat

### 2.42.24 Build a matrix with a function : `makemat`

`makemat` takes three arguments :

- a function of two variables `j` and `k` which should return the value of aj,k, the element of row index `j` and column index `k` of the matrix to be built.
- two integers n and p.

`makemat` returns the matrix A=(aj,k) (j=0..n−1 and k=0..p−1) of dimension n × p.
Input :

`makemat((j,k)->j+k,4,3)`

or first define the h function:

`h(j,k):=j+k`

then, input:

`makemat(h,4,3)`

Output :

`[[0,1,2],[1,2,3],[2,3,4],[3,4,5]]`

Note that the indices are counted starting from 0.