I/O Runtime Wishlist

There have been a number of feature requests regarding runtime I/O. This issue is meant to collect them all into one place so we can manage them.

- [ ] Check if list of output variables is valid - see #2246
- [x] Compare ATM `dt` with the frequency of output. This is an issue when the output frequency may be smaller than `dt`.
- [ ] Make sure no two output streams will share a name (i.e. overwriting each other). This is a pretty edge case, so not high priority.
- [ ] Make sure all required keys are present. i.e. filename_prefix, grid, fields, frequency, etc.

Start with the remaining checklist in issue #2939 and review the related issue #2246. Inspect the runtime I/O implementation and its existing validation paths; done means the listed output-variable, stream-name, and required-key checks have been addressed.

Written by the indexing model from the issue text.