Changed default twelve-tone row inversion. CHANGE.

OLD: twelve-tone rows inverted pitch-classes around pitch-class 0.

NEW: twelve-tone rows invert pitch-classes around first pitch-class in row.

Explanation: the change accords with the historical way Schönberg and his
circle inverted their rows.

Note that there is now an axis=None keyword implement against
TwelveToneRow.invert(). When axis=None the row inverts around the first
pitch-class in row. To get the old behavior back set axis=0.