Minor attribute fix

[Vindaar]

Just a couple of minor fixes for some gotchas when reading attributes. Both [] taking a string (returning a data type kind) and withAttr would previously fail, if the attribute hadn't been read at all. Now we make sure to actually check in the file first.

* v0.6.3
- make ~[]~ for attributes taking a string only for the data type kind
  to check if the key exist, raise if not.
  Previously if the attribute hadn't been opened before, the call
  would fail even if it exists in the file
- make ~withAttr~ check if the attribute exists in the file
  first. Previously would fail if it hadn't been opened before