Allow multiple groups in para shell mode

[rackberg]

The para shell command should allow to add multiple groups to enter the para shell mode.
This implies that a the user should be able to exclude a group using the "-x" option.

The shell commands should be executed for each project of every project group specified.
Even if the same project belongs to more than one of the groups the shell uses for command execution, the project should be used once to avoid duplicate execution for a project.