[restraints] define restraints by componentA/B instead of host/guest

[IAlibay]

Follow up from #1043 - specifically a comment by @hannahbaumann on the usage of host/guest when it comes to defining a flatbottom distance restraint between two molecules (which doesn't make sense).

It may be best if we instead rename everything to be component A/B instead of host/guest. This might need more thought though, so let's aim to refactor things once #1043 is done?