Reorganize IntermediateTerm slightly

[mortenpi]

We also need seniority (or something more general, #51) quantum numbers also for jj-coupled CSFs. So, unless I am missing something, I propose:

• Making IntermediateTerm parametric (term could be either Term or HalfInteger)

• Removing the IT type parameter from CSF, i.e. we'd have

  struct CSF{O<:AbstractOrbital, T<:Union{Term,HalfInteger}}
    config::Configuration{<:O}
    subshell_terms::Vector{IntermediateTerm{T}}
    terms::Vector{T}
  end

I could look at tackling this, and also maybe fix #50 while I'm at it.