MSCW-reader reads in trends of all available year rather than only years selected with periods

[heikoklein]

Describe the bug

• Pyaerocom version: latest
• Configuration file (if applicable):
  • cfg_emep-status_trends.py.gz
• Error message (if applicable): On a 2 year = 24 timestep monthly evaluation, 48 timesteps (= all in subfolders) are read

Extracting timeseries data from large array (shape: (48, 520, 1200)). This may take a while...

To Reproduce
Steps to reproduce the behavior:

1. create a trend-analysis with mscw-reader. Select periods to be smaller than available trend-years.
2. The mscw-reader will try to read/collocate all available trend-years, not only the necessary once.
3. pyaerocom takes much longer to evaluation (typically, only every 5th year are selected) and crashes due to timeouts.

Expected behavior
The mscw-reader only reads years selected by periods.

[lewisblake]

This is probably worth discussing. It might fall under the class of a "feature" and not a bug. Trends were originally defined to only exit over seven year periods (or more), so this may have been intentional. The statistics become less robust if we consider short time frames, but maybe there is a specific use case you have in mind.