feat(merge): add pangraph merge command

CHANGELOG.md

@@ -1,3 +1,12 @@
+## Unreleased
+
+- added `pangraph merge` command, to combine two existing pangenome graphs into a single one, see #197.
+- `pangraph build` now accepts a single input sequence.
+- more strict checks on input sequence ids: duplicated or empty ids are now rejected. The `--verify` options compare the input sequences by id and not by order.
+- more strict graph validation at loading.
+- block and node identifiers are now derived from genome names instead of the order the input sequences are read in, enforcing order-independence.
+- fixed a race condition in `pangraph build` that could produce slightly different graphs on repeated runs over the same input.
+
 ## 1.3.0
 
 - added `--guide-tree` option to `pangraph build` command to allow users to provide a custom guide tree in Newick format, see #180.