v3.3.0

Modify measurement-noise interface to clarify conventions:

• previously we would add EFAC + EQUAD noise separately (MeasurementNoise(efac=...), EquadNoise(log10_equad=...)), resulting in total variance EFAC^2 toaerr^2 + EQUAD^2 (the "temponest" convention);
• from now on, we will use the combined object MeasurementNoise(efac=...,log10_t2equad=...), resulting in total variance EFAC^2 x (toaerr^2 + EQUAD^2) (the tempo/tempo2/pint convention), note the change of variable name;
• EFAC noise alone is obtained with MeasurementNoise(efac=...);
• the old convention is obtained by adding MeasurementNoise(efac=...) and TNEquadNoise(log10_tnequad=...).