Prevent too large <backup> values in PartStaffExporter

[jacobtylerwalls]

The PartStaffExporter was producing <duration> values under <backup> tags that were too large by summing the durations of all chord members (instead of just taking one of the durations).

Adds a regression test failing on master:

======================================================================
FAIL: testBackupAmount (__main__.Test.testBackupAmount)
Regression test for chord members causing too-large backup amounts.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jacobwalls/music21/music21/musicxml/partStaffExporter.py", line 1079, in testBackupAmount
    self.assertEqual(
AssertionError: '30240' != '10080'
- 30240
+ 10080


----------------------------------------------------------------------
Ran 21 tests in 2.173s

FAILED (failures=1)

[coveralls]

Coverage increased (+0.001%) to 93.023% when pulling 95ef0de on jacobtylerwalls:too-large-backup into 1a0ec50 on cuthbertLab:master.

[mscuthbert]

Thanks for the fix!