Add getDiag(), getTriu(), and getTril() methods for matrices.

[jacobdwatters]

• getDiag() should return the diagonal entries of a matrix as a vector.

• getTriu() should return the upper triangular portion of the matrix with the rest of the entries zeros.

• getTriu(int idx) should return the upper triangular portion on and above the diagonal idx with the rest of the entries set to zero. Where idx=0 indicate the principle diagonal, idx=1 indicates the first super diagonal and idx=-1 indicates the first sub diagonal.

• getTril() should return the lower triangular portion of the matrix with the rest of the entries zeros.

• getTril(int idx) should return the lower triangular portion on and below the diagonal idx with the rest of the entries set to zero. Where idx=0 indicate the principle diagonal, idx=1 indicates the first super diagonal and idx=-1 indicates the first sub diagonal.