fix non-corrected vcv + newey_west default lags + formatting
fix non-corrected vcv + newey_west default lags + formatting

vcv without correction was incorrectly vcv = σ² * (X'*X)\eye(K) and
should have been vcv = σ² * ((X’*X)\eye(K)) =  σ² * ((X’*X)\I). To
avoid confusion I reverted back to using `inv`, which uses the same
code (I think) as `\I`.
formatting:
* julia uses 4 spaces for tab
* space after commas
* use unicode Ω
* return statement not necessary

Ken-B/AdvEconometrics