Add pop_to function

[metasoarous]

The nestly.scons.SConsWrap.pop method allows you to specify a name argument, which indicates the nest level you would like to pop off (it being understood that you also pop off any and all nest levels between the current nest level and that specified by name). However, it would be convenient to additionally have a pop_to method (or some such; pop_till, whatev) which would pop off all nest levels, excluding the specified name. This allows you to somewhat more easily/consistently pop to a specific nest level, so that you can (e.g.) aggregate some data there, without having to jump through so many hoops about "which nest level is above the one I want to pop to?"