Remove restriction that prevents logging spherocylinder shape

[janbridley]

Description

While the integrators allow for sphereopolyhedra with two vertices (spherocylinders), an internal assertion in ShapeSpheropolyhedron.h::getShapeSpec prevented these shapes from being logged. This restriction has now been removed.

How has this been tested?

Existing tests have not been modified.

Change log

- Spheropolyhedra with two vertices can now be saved as a valid `gsd_shape_spec`

Checklist:

• [x] I have reviewed the Contributor Guidelines.
• [x] I agree with the terms of the HOOMD-blue Contributor Agreement.
• [x] My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.