use AST parsing instead of string parsing to deal with conditionals

[mjskay]

Might need to replace string-based parsing in aes_to_mtx() with an AST-based approach at some point to make it more reliable. Worth looking into making some test cases to break the current implementation.

[mjskay]

E.g. I haven't tried it but my guess is that if a variable name had a pipe in it it would break. E.g.:

parse_aes(aes(width = P(`some|var|with|pipes`)))

[xiaoyingpu]

The good thing is that parsing bits currently don't like this kind of prob var with multiple pipes

These guys work

[mjskay]

ah great! I will have to see if I can come up with other ways to break it then ...