Consider using the JSON schema file name as default namespace

[glopesdev]

In practice for complex projects we always require namespaces in files when processing multiple JSON schema files. To prevent name clash the best practice is to provide an explicit namespace when calling bonsai.sgen. However, this can be error prone and hard to remember. The current default namespace is also not great.

A better default might be to simply use the name of the .json file (without extension) as the default namespace name if no explicit namespace is passed in the CLI. That would greatly improving the experience and also prevent nameclash as long as all .json files are placed in the same folder (since no two files can have the same name in the same folder).