Coin reward override

This PR allows one to set a reward attribute within a coin definition. This is helpful for certain coins whose `coinbasevalue` is not correctly divisible by 100000000, which makes profit checking inaccurate. If no reward attribute is set in a coin, the value will be determined by the block template as previously intended.

Indeed, coin rewards must be updated for block halving or other reward changes. However, as this is merely for profit estimation, I felt it was an appropriate compromise.