Replace reference epoch constants with TimeScale's

[ChristopherRabotin]

As of #289 , TimeScales have a reference_epoch const fn. Yet, the code still references the GPS_REF_EPOCH and other such constants. The purpose of this chore-like task is to replace all of these constants with the reference epoch.

In theory the code should work exactly the same.

This should also document in development section of the README how to add a new time scale.