AST for multiple consecutive operations is incorrect

[jdidion]

Int x = i + i + i

Expected: ExprAdd(ExprAdd(ExprIdentifier(i),ExprIdentifier(i)), ExprIdentifier(i))

Actual: ExprAdd(ExprIdentifier(i),ExprIdentifier(i))

Possible fixes:

1. Generate nested structure as above
2. Make the value of operation types that may be chained a sequence rather than just a and b - this is nicer at least for formatting, because otherwise I need to detect consecutive uses of the same operation and merge them together

[jdidion]

I added branch bug/16-chained-operations with a test case

[orodeh]

I am working on it.

[orodeh]

So far, I've removed potential problems from the scala parser, and found out that if you add braces then things work as expected. So, (i + i) + i works, but i + i + i does not. I have asked Patrick if this is a problem in his wdl4j (WDL for Java) code, which uses the same parser.

[orodeh]

A fix is on master branch.