Add a check if the minimum required version of EnPT is installed.

When updating enpt_enmapboxapp, it might happen that the processing breaks because the GUI already implements a feature that is not yet available in the installed EnPT code, because enpt has to be updated manually.

Proposed solution:

• add a check if the minimal needed version of EnPT is installed, if not, display a warning
• add a hint regarding the minimal required EnPT version that is displayed when enpt_enmapboxapp is updated