new constants ALL_FLAGGED

its odd that to_mask=UNFLAGGED is often our default, because it implies that values that has no flag gets masked / filtered.

i vote for an alias ALL_FLAGGED=-np.inf or even the simpler ALL=-np.inf..

also we should provide a simple alias for no masking at all. since we abandoned True/False for to_mask we should find a way to disable masking..

some suggestions all/none:

• to_mask=ALL_FLAGGED, to_mask=NONE_FLAGGED
• to_mask=ALL, to_mask=NONE
• to_mask=MASK_ALL, to_mask=MASK_NONE
• to_mask='all', to_mask='none'
• to_mask='all', to_mask=None (this needs some adjustment of existing code, because we use None as a default, but this will go anyways see #258 (closed) )