move `addrList` allocation into `setNodeID()`

[2bndy5]

I'm only pursuing this proposal because it would obsolete the private member addrMemAllocated. Initially, this idea came to me (in #220) when discovering that static addresses have to be assigned after calling begin(). In order for a network administrator to give priority to statically assigned addresses, they'd have to assign the addresses before calling update(); this proposal won't change that. However, the requirement for calling begin() feels a bit cumbersome as the addrList array has nothing to do with initializing the required hardware.

I can't think of a reason to keep the allocating code in begin(), so maybe there's a historical reason about why it was written the way it is now.

The proposed code change is pretty straight forward as mentioned in the issue title.

[2bndy5]

I can't think of a reason to keep the allocating code in begin()

I just thought of a reason: Allocating the array is dependent on the ID number passed to setNodeID(). I'm not sure if this proposal is feasible now.

[2bndy5]

Maybe we can move the allocating code into setNodeID(). As for obsoleting the addrMemAllocated member, we might be able to check if the pointer is null instead.

[TMRh20]

I think those are probably both good ideas.