Extend the NGL Viewer plugin for multiple molecule file reading

[wverastegui]

Problem:
The NGL Viewer Plugin lacks the ability to read multiple molecule files, increasing inconvenience when working with distinct molecular dataset

Specific Use Cases:
Users struggle when trying to check groups of molecules from files with multiple entities, particularly before running batch jobs with tools like QCXMS.

Alternatives:
Enabling the plugin to read multiple files would simplify the process, streamline workflow, and enhance the user experience compared to visualizing molecules individually