Implement the Allowed non SI units

R020 BIPM SI units (non-SI units allowed with the 8th edition of the SI) in the quality class BRONZE are

\angstrom,
\atomicunittime
\atomicmassunit
\bar
\barn
\clight
\electronmass
\elementarycharge
\knot 
\mmhg
\naturalunittime
\nauticalmile
\hartree
\bohr
\planckbar