Best approximation splines

for the ISA temperature and pressure and for the powerCurve.
The powerCurve is (unsurprisingly) ill-conditioned, so the error with 400 intervals is 7e-4. The other two have errors on the order of 1e-7 with 200 intervals.
Please review e8d56f72bd957307e5362c239f2116502a45f23a.
I haven't tested this, so I might have made a couple of typos of my own.
Note that I again used pairs of spaces instead of tabs. Tell me if you want me to retab (and with which settings) though I don't think tabs are a good idea for tables like our curves (depending on the width they are set to, things can look very weird).
I have not added the *Mathematica* notebook, I'll wait until you reorganise the repo.