Fixes laptop power draw, improves cell chargers

I thought the fix for laptop power draw was included in PR #5619, but somehow it got lost.

* Laptops no longer draw 200 times as much power as they should, due to missing unit conversion
* Cell chargers no longer call use_power() in their process()
* Converted cell_charger.dm to absolute pathnames

Laptop battery capacity should probably be reduced by 200 to account for the reduced power draw. I have not done this in this PR though as they are currently using the default line of power cells. Really we should probably add a distinction between the industrial-sized power cells used by APCs and Cyborgs (they are w_class 3... it doesn't make sense that they are able to fit inside a laptop, much less a stun baton), and the ones used by devices.