Add support for Soul Core stat requirement conversion

[haugenc]

Fixes #153 .

Description of the problem being solved:

Rune: Soul Core of Cholotl
Rune: Soul Core of Zantipi
Rune: Soul Core of Atmohua
do not properly redistribute stats on gear.

Created mod in special mod list and updated logic in Classes/Item.lua to process flag and apply convert calcuations to base.

Steps taken to verify a working solution:

-Added custom item on Expert Barrier Quarterstaff base with all three runes and matched values to in-game calculation
-Added custom item on Expert Tribal Mask base with one rune + reduced Attribute Requirements and matched values to in-game calcs
-Matched Attribute Requirement calculations to items equipped
-Test case added to TestItemParse_spec.lua

Link to a build that showcases this PR:

https://maxroll.gg/poe2/pob/lt5vh0et

Before screenshot:

After screenshot:

In-game Calcuations for items

[haugenc]

Just a note about the development of this. I initially tried parsing by standard mod text, but couldn't get it to apply to base properly so the attr reqs scaled correctly to in-game values. Also tried adding a mods with flags of PercentStat of the relevant attr reqs, but calcLocal didn't seem to parse any mod in baseList where [1] exists.