Difficulty in estimating fees for rawissuance

The test used for issuing assets using rawissueasset require creating a transaction first that has a fee rate high enough to pay for the fees (50x).

(https://github.com/ElementsProject/elements/blob/d91380384dbd33c7671d7e6c14c120782ccd1710/test/functional/feature_issuance.py)

The way to work around this currently is to try various fee rates and issuances and find the one that is the lowest. It would be nicer if the API just figured out how to handle fees as part of the rawissueasset call.