started on adjustment costs

- hard to make work in flex prices
- "works" in NK setting, but no persistence
- made fully "nominal" version in NK model, but need to resolve
indeterminacy of price level
basic version with price adjustment costs
endogenous capital (needs checking)

- results are strange
- documentation is pretty messy
preparing merge

- undo unnecessary changes to GE_NK (for now, maybe replace GE_NK with
GE_NK_adjcost later)
- update gitignore
- added persistence in mod file, and the no-arbitrage eqn in the doc