Expose KDE alignment frequencies through augur frequencies interface
Expose KDE alignment frequencies through augur frequencies interface

KDE alignment frequencies have been implemented for a while, but they have never
been exposed to the augur frequencies command line interface. This commit allows
users to specify the `--method kde` flag while passing multiple sequence
alignments to augur frequencies and all other flags related to KDE-based
estimation. Only auspice-formatted output is supported here.

nextstrain/augur,blab/nextstrain-augur,nextstrain/augur,nextstrain/augur