Enumerations for _Sample_condition_variable.Val_units (An open enumeration)
`atm` | enumeration_text
`bar` | bar 1 bar = 10^5 Pa
`C` | enumeration_text
`cP` | enumeration_text
`K` | Kelvin
`M` | enumeration_text
`m^2/s` | enumeration_text
`mbar` | millibar
`mM` | enumeration_text
`mmHg` | enumeration_text
`Not defined` | enumeration_text
`P` | enumeration_text
`Pa` | Pascal 1 Pa = 1 N m^-2
`Pa.s` | enumeration_text
`pH` | enumeration_text
`Torr` | enumeration_text