fixed sqrt(0.0) which was incorrectly being flagged as error_sqrt_arg_neg

Fixes #5

Author: jacobwilliams

src/function_parser.f90

@@ -753,7 +753,7 @@ subroutine csqrt_func(me,ip,dp,sp,val,ierr)
     real(wp),dimension(:),intent(in) :: val   !! variable values
     integer,intent(out)              :: ierr  !! error flag
 
-    if (me%stack(sp)<=zero) then
+    if (me%stack(sp)<zero) then
         ierr = error_sqrt_arg_neg
     else
         me%stack(sp) = sqrt(me%stack(sp))