Fixing misc bugs

[zhucaoxiang]

There are several fixes to the bugs I found when using the code.

[zhucaoxiang]

@thomasgbkruger I added some lines to avoid bugs in some special cases, like

1. when Npert = 0 (now the default value), sbnormal related variables will not be saved.
2. when using circular coils, derivatives in nissin will get divided-by-zero errors.

I will merge it for now (some one wants to use it now), but we can create other PRs if necessary.