Fix maps iteration order error in append_test, merge_test, and graphviz_test

Fixes #53 . I fixed `merge_test` and `append_test` by using the maps as a reference of presence and appending directly to the array. It does not add any memory or time complexity.

`TestWriteDot` was fixed by comparing line-wise rather than with order. I am not certain whether order matters in the graphviz output. If it does, the implementation of `BasicParse` would need to be altered.