Replace atomeye with an active open source structure viewer as the default viewer #114

Problem

Currently the app anyeye is using atomeye as the default structure viewer. There are also dedicated modules and functions solving for atomeye xcfg format. However as atomeye is old and hard to install now, we should shift to a more up to date and active open source structure viewer as the default viewer of anyeye app.

Proposed solution

Decide the viewer and modify the plugin. Potentially remove xcfg support. Update anything related in docstring and documentation. After this add anyeye as a terminal script in pyproject.toml.