Fixes to tests and package checks

Closes #53, #52, and #54

See merge request !33

According to CRAN rules all exported functions must specify a return value.

Update readme

Closes #47

See merge request !32

Release candidate

See merge request !31

version 3.0.0

Closes #24, #27, #36, #37, #38, #39, #40, #41, #43, and #45

See merge request !30

closes #38

v2.7.0 - return stresses of predict_mixture()

See merge request !29

Improve ec() and axis concentration handling

Closes #35

See merge request !28

Replace the "use_for_plotting" column in mod$curves with the plot axis concentrations, which is far more useful.
Add "refence" argument to ec() and some minor improvements.

Some bugfixes and more flexible concentration lengths

See merge request !27

Control whether observed points are plotted

See merge request !26