Only read files fullfilling certain convention

[heikoklein]

Is your feature request related to a problem? Please describe.
pyaerocom currently reads all files it finds and tries to operate on them, which is often time consuming. On the other hand pyaerocom has internal undocmented file-name conventions (metastandards). These conventions should be well documented and enforced.

Describe the solution you would like to see
A clear and concise description of what you want to happen.

• Good documentation of all available conventions
• Only files meeting these conventions are read, other files are mentioned in the logs as INFO.
• Performance should increase by reading less unnecessary files.

See also #1067