Do not always concatenate all files

[gold2718]

When creating yearly averages, it is often desirable to keep some monthly files separate.
Currently, these files are averaged into yearly files.
For the example given by @DirkOlivie, this could be implemented with a --keep-monthly option.
One question is how to determine a monthly file:

-  A filename with only year and month in the filename (e.g., `1850-04`)?
- Something inside the file (e.g., one frame with a period that covers the month)?

Another question is whether there might be other filters for files to be compressed but not concatenated.