When open_ncml is called with group="*", every group will be read
and they will be flatten in the resulting dataset.
If names are conflicting, the dimensions and varaibles names are
appended with __n where n is the number of existing simlar names.

- improved performances by parsing ncml reprsentation only once
- fixed issues of rewritting content when read multiple times

Co-authored-by: David Huard <[email protected]>