units-synth produces rational power that units-check cannot parse

[dorchard]

e.g.

program foo
  != unit X :: x 
  real :: x, y

  y = x ** 2
end program foo

then

$ camfort units-synth foo.f90 foo.f90

Synthesising units for 'foo.f90'
Writing foo.f90

foo.f90:
  3:11 unit x :: x
  3:14 unit x**2.0 :: y

$ camfort units-check foo.f90 foo.f90

Checking units for 'foo.f90'
Error (3:3)-(3:22): Not valid unit syntax at ".0) :: y"
foo.f90: Consistent. 2 variables checked.

We need to output a rational not floating point form.