2024-10-10: PMT Recursive Variants

[RalphSteinhagen]

• 3 options
  -- recursive
  -- encapsulated unique ptr
  ---- like rust
  ---- gets rid of the recursion
  -- std::any

• Tradeoffs
  -- runtime performance, syntax, LOCs in pmt

• proof in the pudding
  -- recommended approach - try the std::any, wrap pmt::get and replace in gr4.0, pmt::visit
  -- see the qa_type_conversions test since it has ~75 combinations of blocks/pmts
  -- most of the changes in the settings, a few in block

• Decision - make api that can swap under the hood these options if compilers advance in the future
  -- (also do the same for std::format vs fmt --> gr::fmt)