Shouldn't be able to add a HowlFieldSchema with the same name as existing

As noted in HowlSchema.java, one should not be able to append an a field schema with the same name. The code says that this requires Comparable, which is not correct, since we're not doing ordering. Technically, this requires a correct equals, but since we don't want multiple fields with the same name in a schema (particularly since we index it via the name in the accompanying map), the correct check is just based on the name. This adds the check and throws a HowlException if this occurs.

Unit test to verify is included.

Additionally, we should make it an invariant in the constructor that the list of field schemas should not include any with duplicated names. I'll do this in another issue.

Good catch.

Went ahead and added check in constructor for field name uniqueness. Might as well do it now.