Add volume normalisation toggle

[MichaelOvens]

Re. https://github.com/mlavik1/UnityVolumeRendering/issues/201 and https://github.com/mlavik1/UnityVolumeRendering/issues/199

Adds a simple setting that can be toggled to control automatic normalisation of imported datasets. This setting has been placed in PlayerPrefs rather than EditorPrefs so it can be accessed outside the editor. This could potentially be expanded in the future to have overrides for different file types (ie DICOM, NRRD, etc). The setting will default to non-normalisation.

Additional minor changes:

• Fixed line height of the 'Settings' header style so that hanging characters (g, p, etc) aren't truncated
• Enclosed toggles in horizontal groups so that the toggle can be on the same line without truncating the label text
• Moved the existing downscale prompt setting from EditorPrefs to PlayerPrefs to match the new normalisation setting

[MichaelOvens]

The setting will currently default to non-normalisation, but we can flip it around to make it default to normalisation if preferred.

I think non-normalisation is clearer, the only danger is if there's some datasets that have erroneously large scales, but I suspect those will be fairly rare and easy to just re-import with the setting toggled.

[mlavik1]

Thank you! This is definitely a good setting to have, and I agree that it makes sense to disable normalisaton by default. My main concern was the RAW datasets, but turns out that the dataset scale is (1,1,1) for those anyway, so they don't change :)

I'm away on holiday now, but I'll get back to the updated other issues/PR once I'm back (I saw you commented on one).

Again, I really appreciate all contributions I'm getting, and you've done a lot for this project so far :+1: