Implement explosive trap +-30% base tertiary radius, fix overlap chance for transfigured version

[Edvinas-Smita]

Fixes #6909 and #7260 .
Supersedes #7321 .

Description of the problem being solved:

Explosive Trap of Shrapnel was using wrong variables for it's overlap calculations and producing incorrect values.
The +- 30% base radius on both explosive traps was not yet implemented leading to overestimating the damage output.

Steps taken to verify a working solution:

• The "Average explosions hitting" setting produces a bit lower overlap chance for normal version and not 100% for trans version

Link to a build that showcases this PR:

https://pobb.in/HFrrt77bxIjn

Before screenshots:

After screenshots:

[Puddlestomper]

I'll just note here that there's a slight inaccuracy in the calculations.

The PR first finds the average radius and then uses that to calculate the overlap chance, but actually you should calculate the overlap chance for each possible tertiary radius and then take the average of those chances (weighted by the chance for that tertiary radius to occur). These are not mathematically the same.

I ran some python experiments to see what difference it makes. It seems to sometimes overestimate and sometimes underestimate depending on increased AoE and boss size. It makes up to a 10% difference in overlap chance (this is with no AoE modifiers and boss radius 3 or 5). I don't want to make the post too long so I won't include the code but I can send it if asked.