Housekeeping: eliminate cruft

[tgperring]

The original Horace dates back more than 15 years, and a number of housekeeping issues have accumulated that make code development trickier, reduce robustness and are potentially confusing to future new project staff. These should be addressed as 'background' tasks running continuously alongside feature development and bug fixing.

This epic acts as an index to tickets that describe specific housekeeping issues

• The helper class horace_paths should be the only source of paths inside Horace code #1885