Reviewer 1-Q2: move some material to intro

[Steboss]

The Introduction describes single vs dual topology in much needed detail. However, the further subdivision of single topology into parameter vs energy scaling is brought up only later (p9). I think this is so fundamental that it should be discussed together with the single/dual topology distinction. These subtle(?) differences see some 'revival', see e.g. 10.1021/acs.jctc.7b01175. I personally tend to classify methods into

dual topology, single topology (parameter scaling) and dual-topology-single-coordinate ("single topology with energy scaling")

[davidlmobley]

Sounds like this should be addressed briefly in the intro.

[jmichel80]

Moved the relevant discussion on p9 (Methods) to p6 (Introduction). Fixed by commit 278