Throw an exception if the user has requested weighted KDE frequencies with
weights that do not match any of the tips in the given tree. This commit
explicitly checks for an empty dictionary of weights after filtering for
representation by tips and raise an exception with a meaningful error
message (instead of allowing the code to continue running and throwing a less
meaningful ValueError when no valid weights remain). This commit also adds a
unit test for this behavior.

Closes #425.

… estimation

Fixes another bug with frequency estimation caused by a mismatch in the casing
of attributes attached to the nodes in the tree, weight attributes in the
weights JSON, and what the estimation code originally expected.