Auto-promote JsonInt to Float when deserializing

[elegios]

Json doesn't distinguish between Int and Float, it only has numbers, but our json library does (to be more convenient with the rest of the system). This means that something that looks like an Int (no fractional part) will be parsed to JsonInt even if our program expected a Float in that position. The easiest way to deal with this is to let the deserialization function used after parsing (jsonDeserializeFloat) automatically promote to Float if it encounters a JsonInt.