Need for variables units

Fixing the units reduces the re-usability of ICAT fields

!63 (comment 322914)

Just like NeXus we can store the units themselves.

The "units" XML attribute becomes

https://manual.nexusformat.org/nxdl-types.html#unit-categories-allowed-in-nxdl-specifications

And this

<beamSize NAPItype="NX_FLOAT" units="micron">${PTYCHO_beamSize}</beamSize>

becomes

<beamSize NAPItype="NX_FLOAT" units="NX_LENGTH">${PTYCHO_beamSize}</beamSize>
<beamSize@units NAPItype="NX_CHAR">${PTYCHO_beamSize@units}</beamSize@units>

or something like

<beamSize NAPItype="NX_FLOAT" units="NX_LENGTH">
    ${PTYCHO_beamSize}
    <units NAPItype="NX_CHAR">${PTYCHO_beamSize@units}</units>
</beamSize>

Or whatever XML allows us to do.