Fields defined in the dictionary which have units

[bruceravel]

This is one of a short string of issues I am opening up after doing some work on the specification document leading up to #28.

Should something like Mono.d_spacing be

  Mono.d_spacing = 3.13543 Angstroms

That is, should the unit be included. This would allow reasonable (and SI!) things like

  Mono.d_spacing = 313.543 picometers
  Mono.d_spacing = 0.313543 nanometers

with a unitless number

  Mono.d_spacing = 3.13543

interpreted as being in Angstroms.

[newville]

I don't disagree with this, but the complication with units can quickly spiral out of control (pm, picometers, picometres, .1e-12 m, 1x10^-9 cm). Is the library supposed to validate units and return values with meaningful units (like, auto-convert to nano-feet) for all numbers, or just Mono d spacing?

Or, is the unit just a string? What unit strings are allowed? What about "Unicode 212B"? Do we need a separate dictionary of units?

There is a real need to have mono d-spacing -- it is used in the format. But having to recognize and act on units could easily double the size of the C function. Allowing one of ("Angstrom", "nm", "pm"), with a default of "Angstrom" might be a reasonable compromise, but that is not anything close to "handling units".