v1.2.3: Bug fix in goodness of fit and other refactors

- Bug fix in `GoodnessOfFit`, correctly use `n_obs_` rather than
  `n_data` when calculating the chi-squared statistics

- Move `.h` files to `include/`
- Add `arma.h` and use `-DUSE_RCPP`
- Add various utility functions in `util.h`
- Overload constructors, making parameters for the regression problem
  optional
- Use `[[maybe_unused]]`
- Add assertions
- Implement `GoodnessOfFit`
- Use `atomic` in `EmAlgorithmArray` and `Blrt`
- Use templates in `standard_error_rcpp.cc`
- Other minor C++ refactors