For #175: Add tests and fixes for twophase(method="simple").

[bschneidr]

This PR makes sure that 'srvyr' works for designs where the user specifies twophase(..., method = "simple").

Everything so far operated on the assumption that users would be using twophase(..., method = "full"), which is why we see the problems faced in #175.

I copied the tests we're using for method = "full" and extended them for the case where method = "simple", which caught the problem raised in #175. To make things work, I added a subset_svy_vars() method for objects with class twophase, and otherwise in the helper functions just updated a couple checks in places to ensure that twophase objects get treated like twophase2 objects wherever possible.

[gergness]

Amazing, thanks so much!