Hi everyone,
I just fully implemented $toDecimal operator of aggregation convertor with UTs.
The original behaviors of $toDecimal can be found in https://docs.mongodb.com/manual/reference/operator/aggregation/toDecimal/#behavior

There is one behavior that doesn't have error detail in document is

You cannot convert a string value of a non-base10 number (e.g. "0x6400")

After I check the real error from MongoDB, I confirm it will raise a OperationFailure with detail. I also implement it with a UT.

If it's convenient, would you like to check and merge this request.

PS： I may found a bug in the base function _Parse when I wrote tests. I will open an issue for it later.

Cheers!