Princurve

* BUG FIX `principal_curve()`: avoid division by zero when the initial principal curve
has already converged.

* BUG FIX `project_to_curve()`: set dimension names of outputted `s` correctly.

* DOCUMENTATION: Added `cran-comments.md` and `revdep` to repository.

* MINOR CHANGE: Removed `adjust_range()`; use `grDevices::extendrange()` instead.

* TESTING `start_circle()`: Added unit tests.

* BUG FIX `start_circle()`: Make sure circle is centered and scaled correctly.

* MINOR CHANGE: Move smoother functions from inside `principal_curve()` to
a list `smoother_functions`.

* TESTING `smoother_functions`: Added tests to ensure each of the smoother
functions work correctly.

* SPEED UP `project_to_curve()`: Significantly speed up this function by not
allocation objects that don't need allocation, and pre-allocating objects
that do.

* SPEED UP `principal_curve()`: Added `approx_points` parameter. This allows
approximation of the curve between smoothing and projection, to ensure
`principal_curve()` scales well to higher numbers of samples.

* BUG FIX: Fixed issues with legacy `principal.curve()` and `get.lam()` (8).

* TESTING: Perform direct comparison between the current princurve and
princurve 1.1-12 to check whether `principal.curve()` and `get.lam()` produce
output with exactly the same format and almost exactly the same values.

* SIGNIFICANT CHANGE: Remove the `ord` parameter from `project_to_curve()`,
in order to reduce the amount of Rcpp code a little bit.

* DOCUMENTATION: Improved citations (10).

* DOCUMENTATION: Improved news (11).

* SPEED UP `project_to_curve()`: Do not compute `lambda` accurately as it
is only used to order the points in `x`.

* MINOR CHANGE: Added `...` argument to `whiskers()`.

* DOCUMENTATION: Minor fix in output documentation of `principal_curve()`.

* DOCUMENTATION: Added more information to the README.

* MINOR CHANGE: Support both `news()` and markdown news on GitHub.

* SIGNIFICANT CHANGES: Reimplemented `project_to_curve()` completely in
Rcpp, thereby completely removing all Fortran code from princurve.

* TESTING: test both on linux and osx.

* TESTING: Added tests to ensure `project_to_curve()` works similar to
legacy `get.lam()`.

* MAINTAINER: Changed the maintainer from Andreas Weingessel
to Robrecht Cannoodt.

* SIGNIFICANT CHANGES: Added functions `principal_curve()` and
`project_to_curve()` with a slightly different interface than
`principal.curve()` and `get.lam()`.

* DEPRECATION: Prepare `get.lam()` and `principal.curve()` for deprecation
planned on 2018-07-01. Will contact maintainers of reverse depending
packages.

* BUG FIX: Allow `start_circle()` to work when the number of dimensions
is larger than 2.

* DOCUMENTATION: Rewrite the README in markdown.

* DOCUMENTATION: Use roxygen2 for the documentation.

* MINOR CHANGES: Clean up code to ensure consistent code formatting.

* TESTING: Added tests for `principal_curve()` and `project_to_curve()`.

* TESTING: Enabled continuous integration using
[travis-ci.org](https://travis-ci.org/rcannood/princurve) and
[ci.appveyor.com](https://ci.appveyor.com/project/rcannood/princurve).

* BUG FIX: src/sortdi.f (sortdi): Fix Fortran array bounds problem.

* MINOR CHANGES: Update for R 2.0.

* DEPRECATION: Removed `whiskers()`