Update maisi readme and add input check to controlnet infer script (#1825)

Fixes # .

### Description
1. We provide several recommended spacing parameters for different
output sizes in README
2. Add an additional input check in the ControlNet inference script to
prevent generating images with small FOV.

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Avoid including large-size files in the PR.
- [x] Clean up long text outputs from code cells in the notebook.
- [x] For security purposes, please check the contents and remove any
sensitive info such as user names and private key.
- [x] Ensure (1) hyperlinks and markdown anchors are working (2) use
relative paths for tutorial repo files (3) put figure and graphs in the
`./figure` folder
- [x] Notebook runs automatically `./runner.sh -t <path to .ipynb file>`

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Signed-off-by: Pengfei Guo <pengfeig@nvidia.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>