Proposal to fix issue #103

Hey :)

Issue #103 (closed) showed the 'default measurement group' implementation was broken. The question of a 'default' measurement group was raised in issue #77 (closed) but we did not have a clear answer. The idea behind the current implementation of the default measurement group corresponds in fact to a proxy to the active measurement group. So this proposal removes the concept of 'default' measurement group in favour of the 'active' measurement group. Indeed, it is quite difficult to have a default measurement group if it is not defined in the configuration.

When there is no active measurement group, and no defined measurement group in a session, an empty measurement group (with no counters) is set as the 'active' one. In this case, ct(0.1) or scans without counters specified explicitly doesn't work, but at least a proper exception is raised.

I also add some tests for measurement groups ;)

We need more work on measurement groups, to have the concepts better defined ; the current API is also not 100% satisfactory (see enable and disable property setters...).

I can't remember the reasons why we cannot just use simple lists of counters, and switch from one list or another when launching the scan commands :-P