Add parentheses for legato based on LilyPond slur syntax

[pinobatch]

"Legato" in Pently refers to an instantaneous change in the base pitch of a note without restarting the note's envelope. Currently in pentlyas.py, it's possible to trigger legato in a pitched pattern by adding ~ to the preceding note. This is derived from LilyPond tie syntax. But LilyPond also has slur syntax, triggered with ( suffix on the note before the first legato note and ) suffix on the last legato note. For example, these should be equivalent, where only the C, F, and G get a new envelope:

c4~ d~ e f g2
c4( d e) f g2

[Qix-]

Awesome 💃