Fix look-ahead implementation in case of biased proposals

[EmadAlamoudi]

Fix the lookahead sampler implementation

[codecov-commenter]

Codecov Report

Merging #586 (f0ac64f) into develop (6446a77) will decrease coverage by 35.39%. The diff coverage is 56.82%.

@@             Coverage Diff              @@
##           develop     #586       +/-   ##
============================================
- Coverage    84.08%   48.69%   -35.40%     
============================================
  Files           91      122       +31     
  Lines         5260     7751     +2491     
============================================
- Hits          4423     3774      -649     
- Misses         837     3977     +3140     

Impacted Files	Coverage Δ
pyabc/cv/powerlaw.py	45.45% <0.00%> (-54.55%)	:arrow_down:
pyabc/inference/__init__.py	100.00% <ø> (ø)
pyabc/predictor/predictor.py	23.79% <ø> (ø)
pyabc/random_variables.py	65.15% <ø> (-0.73%)	:arrow_down:
pyabc/sampler/__init__.py	100.00% <ø> (ø)
pyabc/sampler/base.py	94.44% <ø> (+0.50%)	:arrow_up:
pyabc/sampler/concurrent_future.py	50.00% <ø> (-50.00%)	:arrow_down:
pyabc/sampler/dask_sampler.py	50.00% <ø> (-50.00%)	:arrow_down:
pyabc/sampler/eps_mixin.py	16.45% <ø> (-83.55%)	:arrow_down:
pyabc/sampler/mapping.py	30.00% <ø> (-65.00%)	:arrow_down:
... and 184 more

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