correct definition of dram, per CEG-308566, and add definitions

[graybeal]

While the fluid dram specification is ok, the plain dram is:

• an avoirdupois unit of mass (and so should not be based on fluid ounces) [1] [2] All references to the use of dram as a fluid measure call it a 'fluid dram' (and make it 1/8 fluid ounces).

This changes the definition was changed and moves the result into the mass section.

[1] National Institute of Standards and Technology (October 2011). Butcher, Tina; Cook, Steve; Crown, Linda et al. eds. "Appendix C – General Tables of Units of Measurement", page C-6 [2] WIkipedia entry https://en.wikipedia.org/wiki/Dram_(unit)

[graybeal]

The first request (af76193) is for the modifications to change fluid dram to avoirdupois dram only.

The second and third commits are to add definitions to all the units in all 5 files, and to move the comments into their own tags or attributes, as appropriate.

Sorry I couldn't seem to squash the last two into a single commit, and I couldn't seem to make this two different pull requests. Still mastering the technology.