Resolve "Consistency of icet/hiphive interface"

Description

• All occurrences of 'size' have been replaced with 'radius'. This includes changes to C++ code, Python modules, and tests. In the process I annotated and cleaned up src/Geometry.*pp.

• get_orbit_list_info was renamed to orbit_data and changed to a property even though it is relatively involved operation that exceeds the "tightness" of what a property constitutes (see discussion with @erikfransson).

• In the process I replaced spaces and hyphens in dictionary keys with underscores in order to enable direct utilization of these data structures in pandas. Also MC vector is now multi_component_vector.

Closes #178 (closed)