Relax type signature to AbstractString for term_id?

[harryscholes]

OBOParse.jl/src/ontology.jl

Lines 34 to 37 in 4c84576

	Base.getindex(ontology::Ontology, term_id::TermId) = ontology.terms[term_id]
	Base.getindex(ontology::Ontology, term_ids::Set{TermId}) =
	[ontology[t_id] for t_id in term_ids]

At the moment this doesn't work for e.g. term_id::SubString{String}

[alyst]

@harryscholes Relaxing to term_id::AbstractString and term_ids::Set{<:AbstractString} seems reasonable for me. Would you like to submit a PR? We also would need to add tests to ensure the relaxed indexing actually works.