Replace d2norm with hypot

[ivan-pi]

The function d2norm defined here

https://github.com/jacobwilliams/LSQR/blob/e5818043de3884ac80b4b54f29097d441e2f2667/src/lsqr.f90#L1164

could be replaced with the intrinsic function hypot which returns a value equal to a processor-dependent approximation to the Euclidean distance, without undue overflow or underflow.