Release of version 0.13.0

- change maintainer
- bump version
- integrate RcppParallel
- move most matrix operations implemented in C++ to parallel execution
(close #18)
- small fixes and refactoring

Fixes for CRAN

- Change the way DOIs are referenced

Add DOIs in description, fix utils / stats imports

- add DOIs for references in the description
- import stats package
- prefix utils function with utils::