Allow geometry transparency at the volume level

[MarkusFrankATcernch]

This Pull request:

This Pull request allows to set in TGeo the transparency of objects at the level of individual TGeoVolume instances.

ROOT was attributing the volume transparency to the material of the volume.
This is not always the desired behavior. The PR preserves the current functionality if the transparency level is not
explicitly set to the volume instance.
Improved functionality: If the transparency level is set directly to the volume instance, the volume transparency
takes precedence over the material transparency. The default behavior can be restored by setting a transparency
level of -1 to the volume.

Changes or fixes:

Improves the flexibility to set transparency to geometry volumes.

Checklist:

• [xxx] tested changes locally
• updated the docs (if necessary)

This PR fixes #

[phsft-bot]

Can one of the admins verify this patch?

[agheata]

Thanks Markus, we need to preserve backward compatibility, see my comment

[github-actions]

Test Results

         8 files           8 suites   2d 6h 33m 34s ⏱️
  2 475 tests   2 468 ✔️ 0 💤   7 ❌
18 846 runs  18 834 ✔️ 0 💤 12 ❌

For more details on these failures, see this check.

Results for commit 0898708.

[agheata]

Failing tests are unrelated