Prevent rounding of the GNSS from nanoseconds initializers

[ChristopherRabotin]

This issue was due to the from_{GNSS}_nanoseconds functions which would round the input nanoseconds from a u64 into an f64. Now we initialize all GNSS time scales using the Duration's from_parts function such that if a duration fits in nanoseconds u64 (i.e. is less than ~4.1 centuries long), then it will be "normalized" and initialized in this time scale.

I added tests to confirm that this fix works for a recent-ish GPST nanoseconds. I could not add tests for any nanoseconds initialization which is more than one century away because the to_{GNSS}_nanoseconds functions return an error if the centuries is non zero (as documented).

Closes #317