refactor flagger methods

i really like to refactor some flagger methods like so:

• setFlagger(other) -> merge(other)
• getFlagger(field, loc) -> shrink(field, loc)

For me its still quite hard, to understand a code-snippet like the following, at a quick glance:

this_flagger = flagger.initFlags(data)
other_flagger = this_flagger.getFlagger(loc=sl).setFlags(field)
result_flagger = this_flagger.setFlagger(other_flagger)

but this is a bit nicer:

this_flagger = flagger.initFlags(data)
other_flagger = this_flagger.shrink(loc=sl).setFlags(field)
result_flagger = this_flagger.merge(other_flagger)

(code-snipped from test_setFlagger())