Fix calculation of basis vectors in convert2cfg.m

* Changed convert2cfg.m to do it's basis vector calculations in degrees Kept all other calculations involving alpha, beta, and gamma in radians (for now) by refactoring the variables to alphar, betar, and gammar, respectively.
    * This prevents very small values arising from (I think) floating point rounding errors in the degree -> radian conversion
    * My initial tests show that it's writing the CFG file correctly now (testing with the BaTiO3 cell and my own 4H-SiC and a-SiO2 data)

* This is the email I sent describing this change:
    * I found that the cos(gamma) call on line 157 of convert2cfg.m was causing very small rounding errors resulting in a very small value for H0(2,1) (something like 2.6e-16). This would be fine, but it looks like QSTEM has trouble reading this in. It's not too much trouble to manually change this to 0 within the .cfg file afterwards, but by switching those two calculations to degrees, I think we can avoid the rounding errors. I've included those changes in a new pull request on github.