Handle epoch for terrestrial measurements

[rogerfraser]

Brief description
A new feature to handle epoch for terrestrial measurements is required.

Related issue(s)
This request relates to issue #170. In particular, when a discontinuity file is loaded, terrestrial measurements containing discontinuity sites are not updated to reflect legitimate discontinuity. This is because epoch is not handled for terrestrial measurements.

Background
The original/default behaviour of DynAdjust for terrestrial measurements does not include any capability to handle epoch. Given the ability for DynAdjust to handle discontinuities, it is now important to be able to update terrestrial measurements connected to stations affected by ground movement.

Basic requirements
Add full functionality to handle epoch for terrestrial measurements.

See the example data files in #170 provided by @nicgowans.

Next steps

• Add methods for GetEpoch and SetEpoch to the relevant measurement files in measurement_types.
• Add functionality to SetMeasurementRec, WriteDynaMLMsr and WriteDNAMsr to the relevant measurement files in measurement_types.
• Update DNA MSR file format to include epoch for terrestrial measurements (columns 123-142, 20 chars wide, format ’dd.mm.yyyy’)

Priority
High priority.

[rogerfraser]

Addressed by #174.