Eig UFF 55 complex value

[thejacksbad]

the definition of eig in prepare55 doesn't allow complex eigenvalue formatting.

Proposed solution:

if not isinstance(eig, (float, complex) and eig != None:

Same approach for modal_a and modal_b

[jankoslavic]

Thank you @thejacksbad for opening this issue. Can you prepare a PR? If yes, please add also a simple test for this enhancements.
If you need help, you can count on me.