Updated isdiag function, new issquare function

Improved cases for sde_isdiag() -especially for scalars.

Added new issquare() function to check if an array is a square matrix. Not used yet.

Signed-off-by: Andrew D. Horchler <adh9@case.edu>
New function to create correlation matrices and correlated data and to also decorrelate them

Particularly useful for generating correlated noise.

Signed-off-by: Andrew D. Horchler <adh9@case.edu>