Precision for units
The unit-conversion part of fdf are using values with less significant digits than what we have in the ISO SIunits system.
In particular the latest SI unit update (in 2019) we now have some values which are exact, by definition.
I could make an initial PR with explicit changes for the system.
Note: once/if this goes in Siesta, we should also update the units in units.F90 ;)