add basic plotting of data

[drbenvincent]

Not convinced it's a good idea to have too much plotting in this package, because it gets difficult to deal with the generality of darc tasks. But for the sake of debugging and really being sure that things are working well, it makes sense to do some elementary plotting of the data (designs, responses) that were run.

Implement this as a method of DARCDesign where all_data is kept.

[drbenvincent]

• [x] better colour coding by response.
• [ ] add a legend for colour coding?
• [ ] scale size of dot by number of instances that design (or x,y point on the graph) occurs?

[drbenvincent]

Note: improvements were made in plotting where there are front end delays in #20

[drbenvincent]

We've not got robust, but basic, plotting working for all the paradigms.

We can certainly expect some improvements here, but for now, we have achieved our goal of visualising the data (designs and responses).