Drdid

* Use qr.solve as default (instead of solve)

* Drop collinear variables in pre_process_drdid.R (useful in drdid command but not other commands)

* Add compatibility with R 3.5

* Improve invertibility of outcome regression design matrix

* Fixed links

* Add new flags for non-unique unit identifier

* Better handle of factor variables as covariates

* Fix issue with NA in covariates

* Allows for treating covariates as factor and alike when computing DiD

* Improve error and warning handling due to collinearity and convergence issues.

* First official version of package, functions for computing a variety of difference-in-differences (DiD) estimators for the ATT.
* Documentation is improved compared to the devel version, including examples for every function now.

* Created wrapper function `drdid`, `ordid` and `ipwdid` to implement doubly-robust, outcome regression and inverse probability weighted DID estimators.

* Add dataset used in the empirical application of Sant'Anna and Zhao (2020).