Read reaction data from the build directory by default

[oparry-ukaea]

The json db location used by reactions defaults to the source directory.
This breaks things when you update the json data, build, then change branch.

Suggested changes:

• Remove HERMES_COPY_JSON_DATABASE_TO_BUILD as a config option and always copy ./json_database to <build>/json_database
• Default to reading from <build>/json_database
• Allow path to be overridden at runtime with the "json_database_dir" config option (already exists)