Physical constants and conversion factors could be refined

[gmagoon]

Some of the physical constants and conversion factors seem to be off from the values I am familiar with by a fraction of a percent. I have already adjusted R (universal gas constant) in the following edit: http://github.com/gmagoon/CanTherm/commit/2a657aea4882337e1c54c8fcf059b32e303afce4 , but various other values could be refined as well. Other examples include:
4180 and 4.18 should be 4184 and 4.184 (J/cal)
6.023e23 should be 6.022...e23 (Avogadro's number)
1.6605e-27 is used sometimes whereas 1.67e-27 is used elsewhere (amu to kg?)