Fixing replication issue with Formula Node

- Compile the formula node to a temp DS Function, which internally call
Formula.Evaluate for actual evaluation.
- Setup a function call for this temp DS function.
Add null return protection

Calling toString on the arguments is causing null ref exceptions. The callers of this method appear to be prepared to handle nulls.
Note what the working directory in RTF should be
Merge pull request #1594 from sharadkjaiswal/Migration

Fixing replication issue with Formula Node
Fix for MAGN-3256, and possibly others.

Repeated failures have been called by in some circumstances to cause a null node to enter the AST.

This was caused by failures in reading the output ports

http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3256
Merge branch 'master' of https://github.com/DynamoDS/Dynamo
Test cases for MAGN_3256