Make `--dir` flag optional in `kernels` subcommands and default to `_data`

[mtardy]

See this thread #184 (comment).

We've been using _data by default for a long time now, it would be nice to default to that folder (and if needed, create it) instead of requiring the user always to enter the same path for each command.

There is also UX standardization that can be done, see this comment from Joe #184 (comment).