Unset number property

[NFarrington]

Similar to #71, it doesn't appear possible to unset a number property as it expects a float64, which cannot be nil. Using an empty NumberProperty sets the value to 0, rather than unsettling it.

Please could it be possible to unset a number property?

N.B. I'm very new to programming in Go, so apologies if there's something I'm missing. I'm learning go with this package in a personal project and it's been very helpful, so thank you 😄