Search for rational surface psis where q=mpols/ntors in interval
[psimin, psimax].
*NOTE* Only the leftmost rational surface will be found.
integer, intent(in) :: mpols resonant poloidal mode
integer, intent(in) :: ntors resonant toroidal mode
real(i2mex_r8), intent(inout) :: psimin lower bound
real(i2mex_r8), intent(inout) :: psimax upper bound
real(i2mex_r8), intent(out) :: psis singular surface
integer, intent(out) :: ier