New simlist

[stefdoerr]

Implement the new class-based simlist which automatically detects structure and trajectory files in the folders. Adaptive would be a nice place to start using it as seen here for example: #255 (comment)
Structures can have arbitrary names (here it's hardcoded to structure.pdb). Should pick by extension and report in case of multiple different structure files.

[j3mdamas]

Done even though without class (Stefan)