Princurve

* BUG FIX `project_to_curve()`: Return error message when `x` or `s` contain
insufficient rows.

* BUG FIX unit tests: Switch from `svg()` to `pdf()` as support for `svg()`
might be optional.

* BUG FIX `project_to_curve()`: Fix pass-by-reference bug, issue 33. Thanks
to szcf-weiya for detecting and fixing this bug!

* BUG FIX: Fix warning in `stats::approx()` due to changes made in R 3.6.

* DEFUNCT `principal.curve()` and `get.lam()`.

* BUG FIX: Removed extra dependencies in princurve by removing vignettes; fixes 28.

* DEPRECATION `principal.curve()` and `get.lam()`.

* DOCUMENTATION: Use the `magick` package to generate animated GIFs in
the vignette, instead of the `animation` package, because
`animation` uses `ffmpeg` which is not installed on all CRAN systems.

* DEPRECATION: Added deprecation which will be triggered starting from 2018-08-01
upon calling `principal.curve()` or `get.lam()`.

* DOCUMENTATION: Added vignettes on the algorithm behind princurve and
on benchmarking results between princurve 1.1 and 2.1.

* BUG FIX `principal_curve()`: Don't apply rownames to curve as approx_points could
be set to a different value other than `nrow(x)`.

* TESTING: Skip comparison unit test between princurve 1.1 and 2.1 on CRAN.

* MINOR CHANGE `project_to_curve()`: Attempt to fix rchk warnings by not using
`x(i, j) = v` notation but instead `x[j * x.nrow() + i] = v`.

* DOCUMENTATION: Fix in README documentation.