Axial symmetry fixes for BlocksOnCylindrical

[markus-jehl]

Changes in this pull request

Fix axial scanner downsampling and correct handling of gaps to make BlocksOnCylindrical axially symmetric.

Testing performed

See before and after in associated issue: #1465

Related issues

Fixes #1465
Relates to #1291

Checklist before requesting a review

• [] I have performed a self-review of my code
• [] I have added docstrings/doxygen in line with the guidance in the developer guide
• [] I have implemented unit tests that cover any new or modified functionality (if applicable)
• [] The code builds and runs on my machine
• [] documentation/release_XXX.md has been updated with any functionality change (if applicable)

[KrisThielemans]

Could you add a note to release_6.2.htm, as well as the tiny comments on the scanner definition in the test function? (If only textual, commit with [ci skip] in the first line of the commit to prevent Appveyor going again).

[markus-jehl]

Projecting a symmetric cylinder with NeuroLF geometry gives only floating point level differences in symmetry (last plot is first axial half minus flipped second axial half of a viewgram):