ParseString errors

[EGAlberts]

Describe the bug
There seem to be fundamental errors in the implementation of parseString inside tree_node.h. Particularly, the find operation is unsafe on L435 if getInput is called during construction. This is because the unordered_map is not instantiated yet. However this is only a side-issue of a more fundamental oversight.

It seems that it is not possible to properly declare your own enum and then parse strings into this, while ignoring the ScriptingEnums functionality. It seems this is a known issue to some extent as there is an exception made for NodeStatus on L433. The direct implication as it stands is that if someone were to use an InputPort for say NodeType, and safely getInput after construction, let's say in tick(), the parsing would be incorrect as it would turn the string provided into an int and then cast that int with NodeType.

How to Reproduce*

I made a fork of btcpp_sample to exemplify the core of the problem.
Here is the output of running the main.cpp there while using the current commit of this repo:

./build/btcpp_sample/btcpp_sample
Inside tick() with getInput:
IDLE
Undefined

Without getInput: 
IDLE
Control

As you can see due to the exemption in ParseString for NodeStatus it comes out correctly, but for NodeType it doesn't.

You can also find commented out code to exemplify the unsafe operation in the constructor.

[EGAlberts]

As for fixes:
The simplest fix for my example is commenting out the else which tries to convert to int on L441, and then avoiding using getInput for this kind of port inside the constructor. You can also of course add a check to see if config().enums exists before blindly trying to find something in it. I'm not sure what the implications would be for someone trying to use the scripting enums in that case. Perhaps it is wholly impossible to use a port with scripting enums inside the constructor right now. I am also not sure if it is desirable or not (in the design) to use getInput at all during construction, but in my case I was misled by earlier success doing so with ports that only reference the blackboard so no user inputted strings.