Deal with front-end delays

[drbenvincent]

• [x] Check code works with front end delays (i.e. DA not just equal to [0])

One of the consequences of this is that we can no longer plot in the same way. My favoured way of plotting data was to plot each design (ie pair of prospects) as a single point. But this is not possible to (as far as I can tell) when there is a front end delay. Therefore we need to do something else...

• [x] when there are font end delays, then we need to plot differently. This will be a figure but where easy prospect is now a point and this will represent simply the prospects tested. It will not itself be indicative of the delay discount function

Also...

• [x] ensure that there are no designs where DA > DB. Do this in generate_all_possible_designs() so that it's just done once up front.

[drbenvincent]

Dealing with designs with front end delays (DA>0) was not a problem, but now we have some testing code for that (NOT proper unit tests).

Plus, we have extracted the data plotting from the DARCDesign class and now we have a little module for plotting of all_data.

[drbenvincent]

What I have now is pretty good. There is plenty of scope to improve the data plotting, but this can happen in good time.