Change schedule.union not to increase depth of schedule tree (#2885)
Change schedule.union not to increase depth of schedule tree (#2885)

* fix #2818; change schedule.union not to increase depth of schedule tree

* Apply suggestions from code review

Co-Authored-By: Lauren Capelluto <2c59feb8d326578951e9ef453f632f3a0bf8c4bd@gmail.com>

* fix timeslots.merged->merge

* keep the implementation out of the docs

QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py