Implement new XP yield function in XP token balanceOf

[0x-r4bbit]

As initial version, we've implemented #53 to determine the XP amount any user has, which also includes the MPs accrued via staking.
This inherently follows an 1 MP == 1 XP relation which is ultimately not what is exactly desired.

After recent discussions, we're now aiming to implement a formula in which the XP yield that is generated by staked SNT decreases, as the amount of SNT staked increases.

Here's what it currently looks like:

Yield per SNT_i = BaseXPperSNT x MP_i / (P x SNT_i)

with:

• Yield per SNT_i - the yield for each SNT staked by staker i
• BaseXPperSNT - the base XP yield per SNT, a constant we decide (and can adjust if needed?)
• MP_i- the MP of staker i
• P- the percentage of SNT staked (total SNT staked / total SNT supply)
• SNT_i- the number of SNT staked by staker i

This yield is still added to the actual balance that the account has in the ERC20 contract