Abstract Decay Chain

[mmikhasenko]

• introduce AbstractDecayChain
• replace over2 by d2
• letterL instrad of printable_l
• improve test coverage

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (824f4a6) 82.62% compared to head (782c38a) 84.35%.

Files	Patch %	Lines
src/decay_channel.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11 +/- ## ========================================== + Coverage 82.62% 84.35% +1.73% ========================================== Files 10 10 Lines 328 326 -2 ========================================== + Hits 271 275 +4 + Misses 57 51 -6 ```

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