Function definition in custom node

- Formula node now compiles to a function definition node, so it can't
be used inside custom node unless we promote the function definition
inside custom node to global scope.
- AstBuilder._CompileToAstNodes() promotes the FunctionDefinitionNode
returned from node.BuildAst() to global scope by notifying the graph
sync data about the function def node and not adding it to resultList.
- Now Custom node supports CodeBlockNode with function definition and
the Formula Node.
Merge branch 'master' of https://github.com/ikeough/Dynamo into Migration
Merge pull request #1625 from sharadkjaiswal/Migration

Function Definition support inside custom node