add units for GNSS total electron content

[BenjaminRuston]

Description

The units for totalElectronContent in the podTC file from COSMIC-2 are TECU so totalElectronContent - Units I presume

Checklist

• [x] I have performed a self-review of my own code
• [x] I have run the unit tests before creating the PR

[BenjaminRuston]

@huishao-r please have a look this is very minor but key one to proceed with