Added wrappers that allow streamlined usage of linear and glm functions

[mbannick]

See NEWS.md file for additional details:

• Added robincar_glm2 and robincar_linear2 functions which allow user more control over covariate adjustment settings rather than the default internal logic for how to specify the working model. Specifically, in robincar_linear2, if users want to include strata as covariates, they add the strata variables directly to covariate_cols. In robincar_glm2, only the "formula" argument is available, rather than relying on the combination of covariate_cols, include_strata_as_covariate, and adj_method as arguments to specify the working model. This is a more direct way to specify the working model.
• Fixed two bugs in the function that parses formulas. Both would have caused breaking errors for the user. Specifically, (1) the previous version couldn't handle parentheses in the formula and (2) if someone had overlapping names for treatment/response variables with covariates e.g., treatment col is "A" but covariate col is "A1", it would incorrectly parse this formula.
• Added a NEWS.md file for tracking changes that is user-facing.

Internally, changed strata to car_strata. This results in a large number of files modified. You can ignore these changes. The files to review are:

• Data.R for the updated formula code (see commit bb50936)
• robincar_glm.R for robincar_glm2() and robincar_linear.R for robincar_linear2() (see commits e4a9fb1, 8a59068 and cb7d83f)
• NEWS.md (see commit 21af579)