`stringsAsFactors = FALSE` - upcoming R 4.0.0.

Dear maintainer,

Please see the problems shown on
<https://cran.r-project.org/web/checks/check_results_protViz.html
<img width="812" alt="Screenshot 2020-02-28 at 09 54 43" src="https://user-images.githubusercontent.com/4901987/75525650-7fe65700-5a10-11ea-850f-fea6a368e608.png">
>.

Specifically, see the problems shown for the r-devel Debian checks.

These can be reproduced by checking with --as-cran using a very current
r-devel (r77865 or later), which makes data.frame() and read.table() use
a stringsAsFactors = FALSE default, which is planned to become the new
default for the upcoming R 4.0.0.

Please see
<https://developer.r-project.org/Blog/public/2020/02/16/stringsasfactors/index.html>
for more information about this change.

Can you please fix your package to work with both the old and new
default?  In principle, this can easily be achieved by adding
stringsAsFactors = TRUE to the relevant calls to data.frame() or
read.table() [or other read.* function calling read.table()], but please
only do this if the sort order used in the string to factor conversion
really does not matter (see the blog post about the locale dependence of
the conversion).  Otherwise, please change to create the factors with
explicitly given levels.

The new problems may be from code in a package you depend on: in this
case, please let me know, and get in touch with the maintainer of that
package.

Please correct before 2020-03-20 to safely retain your package on CRAN.

Best,
-k
cpanse / protViz

`stringsAsFactors = FALSE` - upcoming R 4.0.0. #11
