Introduce a __LABELS group in the meta ini file and propagate the information to the build system

Test labels are a good way to define the interface between the local buildsystem and an automated testing environment. Therefore, tests should be labelled to fall into categories. It would be nice to do this in the meta ini file itself:

[__LABELS.TESTTYPE]
... a set of rules

This snippet defines a label group TESTTYPE. A set of rules should choose one label according to the previous set values. An example for such a label group would be {WEEKLY, NIGHTLY, CONTINUOUS}.

The exact syntax of rules should be discussed after evaluating the technical difficulties in parsing. Possbile example is to allow simple if/else structures in the following C-style form

[__LABELS.TESTTYPE]
<Condition> ? <LABEL1> : <LABEL2>

An example could be then

[__LABELS.TESTTYPE]
{level} >= 5 && {permeability} == tensor ? NIGHTLY : CONTINUOUS
{gridtype} == ug ? CONTINUOUS : WEEKLY