Add stub github action for testing

Actual development will happen in
<#158>
but for testing purposes we need some copy of the workflow on the master
branch. (I know there's another way involving a deliberately broken
workflow on a branch, but I can't find the specifics at the moment.)

Author: jameshadfield

.github/workflows/genoflu-gisaid.yaml

@@ -0,0 +1,7 @@
+# this workflow is a stub action to allow testing from a branch
+
+name: Run GenoFLU on curated GISAID data
+
+on:
+  workflow_dispatch:
+