Some precision improvements for serial kernels

[ptim0626]

This PR contains several fixes that improve the precision in some serial kernels to match those in the normal implementation.

• the same epsilon is used to avoid division by zero, as in here
• use PtyPy's abs2 to calculate af
• do not re-calculate af by fdev + mag, but storing it
• use the exact same formula to propagate as in here

The operations above are mathematically the same before and after the changes, but associative property does not generally hold for floating-point arithmetic, so the above changes ensure the implementation in serial kernels can get the same precision with the normal implementation.

EDIT: Memory is probably more important than precision here, so we don't use additional storage for af.

[ptim0626]

Close for now as each of the suggestions needs to be further investigated, will be addressed separately.