Retrieve only the number in the cdo version. Exclude the letter part.

Hi @nperez

This is fix for the issue #250 (closed) from the external users before. The cdo versions in Ubuntu may be numbers + letters, but the original codes don't expect the existence of letters, therefore, the subtraction of the string caused problems.

The bug has been fixed according to the reporters' suggestion.

Cheers,
An-Chi