feature request: have -mixture accept JSON format

[yarden]

Hi all,

The -mixture option lets you read initial conditions for KaSim from a file. But it looks like -mixture (in KaSim v4.0.94-kappy-85-g0aafc853c) only accepts .ka files.

In the spirit of supporting both JSON/KA, as KaSim does for snapshot output, it'd be great to allow -mixture to take in .json and .json.gz. All my snapshots are in .json format and it'd be nice to just be able to pass a snapshot without converting it to .ka first. Thanks!