Add Join Node

[JonnySpruce]

Node description:

The Join Node combines data from two different streams, where the streams may be of different types.

Development required:

• Similar to Add Merge Node #10, will need to update logic on finding connections between nodes.
• However, there will have to be support for detecting multiple input types to the node (currently the code assumes only one input type)
• Error checking: Node-RED also doesn't limit the number of input nodes, so may need to check there are not more than two input nodes.

Work checklist:

• Add node to the StrIoT Nodes repository
• Add support for the node in the StrIoT GUI repository

Link to relevant StrIoT documentation:

https://github.com/striot/striot/blob/master/docs/Operators.md#join