plotting changes, minor bug fixes, test changes for R 4.0

[TimTaylor]

This PR does the following:

• Generalise the plot function to accept a y-label and graph title. Hopefully the use of null arguments will mean this is not a breaking change.
• adds a new constructor function within build_projections that will hopefully make it a little easier to create subclasses if we want to in future.
• fixes bug that caused using str on a projections object to error.
• fixes bug in regards to subsetting by dates with the [.projections function. Note this was never tested for but was described in the docs. I've not implemented any checks yet.
• added a todo in regards to documenting that build_projections can be used on a df. This was something that was tested for but not documented so I was unsure if you wanted it highlighted - hence the todo.
• I've included some fixes for test errors caused by the move to R4.0. I think these are only intermediate fixes as once you are satisfied behaviour is the same you could probably rebuild the .rds with the current R version. This would also allow you to drop the RNGversion("3.5.3") dependency.
• related to above changes to NAMESPACE, DESCRIPTION and .rd files.

Note - I have been unable to install vdiffr on my local machine so have not considered the image tests.

Changes are grouped by commit so hopefully easy to follow and disregard any that you don't like.

[thibautjombart]

Thanks for the PR!