tile_has_refuel_extra() to use refuel_bases cache
tile_has_refuel_extra() does not use unit class refuel_bases cache. It should.
While at it, rename 'refuel_bases' as 'refuel_extras' as that's what it now is.
tile_has_refuel_extra() does not use unit class refuel_bases cache. It should.
While at it, rename 'refuel_bases' as 'refuel_extras' as that's what it now is.