Grindstones should be used to boost damage by .5 when combining weapons.

To make grindstones have extra function, grindstones should be used to boost weapon damage by .5 when combining two of the same weapon. Can be stacked with sharpness but grindstone damage boost can only be done at a max of 4 times, maxing out at +2 bonus damage. Costs a single digit, fixed amount of experience levels to do it for balance. Allows the player to enhance their favorite weapons a little bit further.