Added new indicator tool: StringContactPoint.

This class accompanies BowHairContactPoint, StringNumber and Tuning in
describing various parameters for string instrument writing.
Added new indicator tool: BowPressure.

This class accompanies BowHairContactPoint, StringContactPoint, StringNumber
and Tuning in describing various parameters for string instrument writing.
Merged master.
Merge branch 'master' of https://github.com/Abjad/abjad