Add support for get_axial_length for cylindrical scanners.

[robbietuk]

Changes in this pull request

Adds support for cylindrical scanners to call get_axial_length() and return a sensible value.

I believe

get_num_rings() * get_ring_spacing();

is the correct implementation over

get_ring_spacing() * (get_num_rings() - 1);

because a scanner with 1 ring will have an axial length of that ring. That is, unless axial bin size should be incorporated into the calculation.

Related issues

Fixes #1493

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)