set_accuracy() refactoring

[Gerard Gorman]

1. set_accuracy() needs to be replaced with set_order(). Make sure that the parameters really do mean order. --- Follow the "Principle of least astonishment"
2. Currently the code fails with an unhelpful message when set_accuracy (to be renamed set_order()). Set a sensible default instead (e.g. 2nd order) and perhaps add some checks to make sure that something sensible is being asked for.

[Tianjiao Sun]

sorted now.

[Michael Lange]

Is this fixed now that PR #47 has landed?

[Christian T. Jacobs]

Yes.