and other unit conversion functions as they are required e.g. slug <-> kg.

- note left utilities/mesh/advanced/collapseEdges/collapseEdges.C as-is.
  It looks suspicious, but the change was recent, so maybe it means something

- can use 'XX.empty()' instead of 'XX.size() == 0', 'XX.size() < 1' or
  'XX.size() <= 0' or for simpler coding.
  It also has the same number of characters as '!XX.size()' and /might/ be
  more readable

- many size checking had 'XX.size() > 0', 'XX.size() != 0', or 'XX.size() >= 1'
  when a simple 'XX.size()' suffices