Missing factor in PricerRule:convertPayCurrencyToToken

[benjaminbollen]

Missing factor in PricerRule:convertPayCurrencyToToken:
https://github.com/OpenSTFoundation/openst-contracts/blob/develop/contracts/rules/PricerRule.sol#L397

misses the factor 10**PriceOracleDecimals. Correction reads:

_payCurrencyAmount
            .mul(
                conversionRateFromBaseCurrencyToToken
            )
            .mul(
                requiredPriceOracleDecimals
            )
            .div(
                10 ** conversionRateDecimalsFromBaseCurrencyToToken
            )
            .div(
                _baseCurrencyPriceInPayCurrency
            );

[benjaminbollen]

above is wrong; UT.decimals must be used. Improved documentation is helpful. The PriceOracle pricePoint is input as u$/OST (not u$/attoOST), which caused the discrepancy. Hotpatched in #161 but leaving this ticket open to re-examine and tidy-up in calmer times