bugfix in mu for pympb

[stevengj]

Fixes #3142, I think. Also a bit of code cleanup by getting rid of the global-like eps member.

@thchr, can you confirm?

[thchr]

Hi Steven; thanks so much for having a look at this and finding a fix..!

I've compiled this branch locally and checked again: everything seems to work now, including anisotropic mu etc.

Many thanks 🙏 !

[thchr]

(my make check reported a fail (but no details) in aniso_disp, but I imagine that's unrelated though)