ENH: Complete the read_castep_new method in CASTEP I/O

The CASTEP I/O module includes a read_castep_new method which replaces the old way of reading .castep files, with the added benefit of reading also forces and stresses and creating a CastepCalculator rather than a SinglePointCalculator to store this information. There is a comment pointing out that the method hasn't been put in place because it still isn't entirely safe against the possibility of calculation_required returning True, leading to any attempt to access these quantities trying to start a new CASTEP calculation. However, from what I see of the two functions, there simply is no way this could happen without the user purposefully modifying something, which is the intended behaviour. Any addition of a special flag would in fact make the behaviour of a CastepCalculator created this way inconsistent and seems to me to be a cure worse than the disease. I'd very much like to have this functionality implemented and am willing to complete it myself, I'm just wondering if anyone thinks an additional level of security should indeed be introduced, and in what respect, or if instead I could simply make a simple edit to replace the current read_castep method with the new one and leave it at that.