(V4): from_mjd_utc / to_gregorian_utc reciprocal

[gwbres]

Hello,

I think I picked up an issue when working with CGGTTS.
In CGGTTS files, Epochs are expressed in MJD UTC. This is how I parse them:

mjd being an integer.
I then use to_gregorian_utc in the mirror operation:

In my example, I parsed and constructed mjd=59567 h=0 mins=14 seconds=0.
Then, dumping generates mjd=59567 h=0 mins=14 seconds=37 (leap second issue ?)

[gwbres]

CGGTTS:dev points to hifitime-4 and is picking up this issue, which seems to be a leap second error that arised.
I will comment out this test to obtain stabilized tools temporarily, but it should be kept in mind