user-defined training_reuse_old_ratio is mandatory

[Vibsteamer]

Revising the default value of training_reuse_old_ratio form "0.2" into "None".
Adding and error message with simple explanations and suggestions.

[codecov-io]

Codecov Report

Merging #343 (f80883b) into devel (d15f199) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##            devel     #343   +/-   ##
=======================================
  Coverage   32.23%   32.24%           
=======================================
  Files          85       85           
  Lines       14290    14292    +2     
=======================================
+ Hits         4607     4608    +1     
- Misses       9683     9684    +1     

Impacted Files	Coverage Δ
dpgen/generator/run.py	69.92% <66.66%> (-0.03%)	⬇️

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