Fire from lenses on sunny days

We can use lenses to start a fire now, during sunny or clear skies.
A magnifying glass item has been added and is craftable, but we can also use lenses, binoculars, monocles and eyeglasses to start a fire.

Firestarting time should take ~1 minute in sunny conditions (60 lighting) and ~25 minutes in clear skies (40 lighting)

Chance to start a fire is 100%, but with 0 survival it takes 3*normal time, with 1 survival 1.5* normal time, while from survival 2 and up its just the base time as described above.
This is to simulate the player failing and trying again until he successfully starts a fire.

I'd like some input about:
1) Do we want to allow lighting fires during clear skies (natural lighting > 40)?

From my experience and a little googling i guess it is possible, but takes a long time, hence why as i made it, it takes 25 minutes at survival 2+ to light a fire via lens. Does this sound ok?

2) In my system the success rate is 100%, but it takes more time at low survival skill levels.
I mean, someone who wants to start a fire, will keep at it until he succeeds, so the failure rates are included into the time spent, to cut down on keypresses.
Do you agree with this?  
If so, shouldn't the way firedrills work also be changed to respect this system? I can do it in this PR if you agree.

3) Do we want to also want allow starting fires with glass shards, glass bottles and water-filled plastic bottles???
Afaik its possible to start fires with all of these, but it does not seem to be adding anything more into the game.

4) Is there a reason we don't have pliers as a tool? Shouldn't they be useful for lots of recipes and actions?



EDIT
this is in response to  #9511
i need it to use the pow() function