Investigate the threshold for parallelism with the poles

[jmert]

The parallelism check coded in

CMB.jl/src/sphere.jl

Lines 13 to 24 in e06c74b

	rtepsone(::Type{T}) where T = sqrt(eps(one(T)))
	"""
	∥(u, v)
	Test whether vector ``u`` is parallel to vector ``v``. Assumes that both are unit
	normalized.
	"""
	function ∥(u, v)
	T = promote_type(eltype(u), eltype(v))
	return (one(T) - abs(u⋅v)) < rtepsone(T)
	end

was chosen relatively arbitrarily. Whether the limit can be made more strict should be investigated.