add numHelper.aboutEqual()

[jasonswearingen]

function aboutEqual( input: number, checkAgainst: number, maxDeviationPercent: number, extraSpreadBuffer:number=0 ): boolean {
    return ( (input * ( 1 + maxDeviationPercent ))+extraSpreadBuffer > checkAgainst ) && (( input * ( 1 - maxDeviationPercent ))-extraSpreadBuffer < checkAgainst );
}