Warn if parsed memory/scratchsize may be too small

[obbardc]

Since the --memory and --scratchsize arguments are parsed with human-readable suffixes into bytes (e.g 2048MB or 2GB), if a user does not put a suffix the arguments are parsed as bytes which can cause issues later when a user sets the memory to be 2048 assuming it will be parsed as MB.

Change the documentation to match reality and also add a warning if the user has set either parameter less than the recommended default.

Closes: #509

[daissi]

Something is wrong with --scratchsize:

./debos ../debos-recipes/rpi3/debimage-rpi3.yaml --scratchsize 100MB -m 100MB
2024/11/14 18:35:32 WARNING: Memory size of 100MB is less than recommended 2048MB
2024/11/14 18:35:32 WARNING: Scratch size of 95MB is less than recommended 2048MB

While is works for --memory, the conversion for scratchsize doesn't give us the same value 100MB vs 95MB

[obbardc]

Something is wrong with --scratchsize:

./debos ../debos-recipes/rpi3/debimage-rpi3.yaml --scratchsize 100MB -m 100MB
2024/11/14 18:35:32 WARNING: Memory size of 100MB is less than recommended 2048MB
2024/11/14 18:35:32 WARNING: Scratch size of 95MB is less than recommended 2048MB

While is works for --memory, the conversion for scratchsize doesn't give us the same value 100MB vs 95MB

I guess that is because for scratchsize we convert to bytes with units.FromHumanSize but for memory we do not :-).

Perhaps we should unify this by:

1. using units.FromHumanSize to convert for both scratchsize & memory.
2. convert to MB using forumula / 1000 / 1000 for both variables instead of / 1024 / 1024. That should make the printed number the same.

To be honest, I threw this together quickly just for some feedback. Happy to rework it.

[daissi]

I would say let's use for both units.FromHumanSize for consistency :) This should avoid confusion.