Improved the Refining spreadsheet

- Applied bootstrap theme.
- Added a special method in the Variation model for this calculations
- Somehow improved accuracy... I just realized the previous version was
  displaying percentages closer to 0 than it should be. So the investment
  returns seemed to be always too little gain or too little loss.

I made the calculations myself this time and compared against the spradsheet's
results and they were the same. Also tested in game and it was pretty close but
not exactly the same since eve-central data is not live.
Fixed and improved Planetary Interaction spreadsheet.

Added some stuff that missed the last commit of this file and then improved
the accuracy of some calculations by taking taxes into account basically
everywhere they apply.