Improve test coverage

[1-Bart-1]

[ufechner7]

An easy way would be to include plotting.jl in the tests. My goal would be 70% for the 1.0 release and 80% for the 1.1 release.

[ufechner7]

Now 64%.

[ufechner7]

Now 71%.

[ufechner7]

Now 83%.

[ufechner7]

Now 85%

[ufechner7]

Most of the work is done, what remains is in #53