added label mechanics to isFlagged / implemented lebelMechanics test

Added label keyword mechanics to isFlagged (as discussed on the phone with @schaefed).

• Implementation is quite non-invasive.
• Biggest change: since history informations must be known to isFlagged - global, the histories get passed on to execGeneric (as opposed to only getting squeezed flags columns)
• added test

Test like so:

dat = pd.DataFrame({'data1': [1, 1, 5, 2, 1], 'data2': [1, 1, 2, 3, 4]}, index=pd.date_range('2000', '2005', periods=5))

qc = SaQC(dat)
qc = qc.flagRange('data1', max=4, label='out of range')
qc = qc.flagRange('data1', max=0, label='out of range2')
qc = qc.flagGeneric('data1', target='data2',func=lambda x: isflagged(x, 'out of range'))

Expected result is, that data2 only is flagged where first call to flagRange triggered flagging for data1.

closes #336 (closed)