Formatting fixes and mean density in the System Editor

[zonkmachine]

Some fixes in physical descriptions.

• Introducing a space before the unit. This was in response to the formatting of objects in the system overview but Format is called elsewhere. I have looked for possible issues in other modules but so far have found none. (Images)
• Quick fix to use m³ instead of m3.
• Add Mean Density to the System Editor. It's good to be able to quickly assess if the mass/volume ratio is within a reasonable
  range.

Before

Fixed

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Some question marks.

• Density should probably have its own format to aid in translation. I backed out of this part for now.
• Mean density, as I first introduce it, is described in kg/m3. It looks like g/cm3 is the way to go for astronomical objects. This is how it's done now in the System editor. At least they should show the same. I'm looking into fixing this for the system overview too. This should possibly be done directly in CalcMeanDensity().

[zonkmachine]

Fixed up gravitational acceleration. I'm not quite sure what changes would be needed in Format for a more permanent solution.

[fluffyfreak]

I think these are good, I don't see any issues with the code 👍

[zonkmachine]

I think these are good, I don't see any issues with the code 👍

Right, I agree. Still, parts of this should probably be fixed in Format but I haven't wrapped my head around it yet. It's not a new issue though and can be fixed up later.

[zonkmachine]

I squashed it down to two commits.