returns a string representation for real r with precision digits and width. The real is correctly rounded to nearest to fit within the precision. Raises an error if r is null.
The minimum possible precision is 1 (automatically set)

Works similar to C/C++ printf, but does not support negative width (left-align with right padding)
Example (Lua):
R2SW(31.1235, 5, 3) --> 31.124
R2SW(1, 5, 0) --> two spaces followed by number
  1.0
See: R2S for a simple converter with preset values