Change "format_slot" to "site"

[trevorbaca]

Abjad's use of (format) "slot" to refer to the lexical position of a LilyPond command has always conflicted with Python's __slots__ class modifier.

OLD:

abjad.BarLine(..., format_slot="before")
abjad.LilyPondLiteral(..., format_slot="before")
abjad.Ottava(..., format_slot="before")

NEW:

abjad.BarLine(..., site="before")
abjad.LilyPondLiteral(..., site="before")
abjad.Ottava(..., site="before")

CHANGED: You must now capitalize LilyPond grob names when using abjad.override(). You must also include # and other Scheme symbols, when necessary:

OLD: abjad.override(...).note_head.color = "red"
NEW: abjad.override(...).NoteHead.color = "#red"