Augment Particle v1 to make it forward compatible with v2

necessary ingredient for Rec#223: adapt Particle v1 to allow uniform treatment with Particle v2.

Basically, use the newly introduced possibility to use ADL to allow classes to 'opt-in' to ThOr functors (Rec!2631 (merged)), and then implement the right (free standing) functions for Particle v1 to customize the implementation of a few key functions.

Note: while Rec!2631 (merged) requires this MR, this MR does not require Rec!2631 (merged). But without Rec!2631 (merged), this MR will not actually do anything noticeable as it just introduces a few new functions.