Record the version within generated modelscan-settings files

[seanpmorgan]

We need to record which version was used to generate a modelscan-settings.toml file so that

• We can warn users that their current settings file may have out-of-date scanners
• We can version our settings file schema and easily handle breaking changes

[seanpmorgan]

Currently this is the behavior for using an old settings file after the scanner re-name:

Detected settings file. Using ~/modelscan-settings.toml. 

Error importing scanner modelscan.scanners.H5Scan
Error importing scanner modelscan.scanners.KerasScan
Error importing scanner modelscan.scanners.NumpyScan
Error importing scanner modelscan.scanners.PickleScan
Error importing scanner modelscan.scanners.PyTorchScan

That should be more informative