Added error function (ERF) routines

[arango]

Added a new file ROMS/Utility/erf.F to incomplete gamma function and error function:

  ERF        Error function, ERF(x)
  ERFC       Complementary error function, ERFC(x)
  ERFCC      Complementary error function, ERFCC(x): cheaper Chebyshev fitting approximation
  GAMMP      Incomplete gamma function, P(a,x)
  GAMMQ      Incomplete gamma function complement, Q(a,x)=1-P(a,x)

This functions are not part of the standard intrinsic functions in Fortran. There are handy when defining analytical expression. The implementation of these functions are adapted from Numerical Recipes.