roots() not working for RatFun

[sverek]

Can not use roots() on a rational function.

MWE:

using ApproxFun,RatFun
function mwe()
	t = Fun(identity,0..pi)
	f = (2-2*cos(t))^2
	g = 2-2*cos(t)
	r = f//g
	display(roots(f-3.5))
	display(roots(g-3.5))
	display(roots(r-3.5))
end

Output:

julia> mwe()
1-element Array{Float64,1}:
 1.5061656879014247
1-element Array{Float64,1}:
 2.41885840577638
ERROR: MethodError: no method matching -(::RationalFun{Fun{Chebyshev{Interval{:closed,:closed,Float64},Float64},Float64,Array{Float64,1}},Fun{Chebyshev{Interval{:closed,:closed,Float64},Float64},Float64,Array{Float64,1}}}, ::Float64)
Closest candidates are:
  -(::Float64, ::Float64) at float.jl:397
  -(::Complex{Bool}, ::Real) at complex.jl:298
  -(::Missing, ::Number) at missing.jl:93

julia> versioninfo()
Julia Version 1.0.3
Commit 099e826241 (2018-12-18 01:34 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i5-4200U CPU @ 1.60GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.0 (ORCJIT, haswell)

[dlfivefifty]

@sverek It looks like Marcus is going to look into this, but note this package is very much a WIP, and it's not getting much attention at the moment. Any help vial PRs would be very much appreciated.