Correctly handle "Concurrent" with Measurement Period Start or End

When the HQMF parser sees references to Measurement Period Start and
Measurement Period End these are converted to Start of Measurement
Period and End of Measurement Period; this commit adds support for the
"Concurrent" operators during this conversion, ie SCW MPE -> SCWE MP
Merge pull request #143 from projectcypress/concurrent_with_measurement_period

Correctly handle "Concurrent" with Measurement Period Start or End