Define the TAU circle constant [Feature #4897]

* Define TAU constant in math.c similar to PI.

* Define TAU method in bigdecimal/math based on existing
  PI method.

* Minor change to error message about zero or negative precision in
  BigMath.PI for clarity and consistency with other methods.
Let me know if you don't like this. I'm happy to leave it as 'argument' if that is preferred.