Add fixation cross between stimuli

[saeub]

No description provided.

[codecov]

Codecov Report

Merging #42 (beb3fa3) into dev (bec1763) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev      #42      +/-   ##
==========================================
- Coverage   75.73%   75.70%   -0.03%     
==========================================
  Files          20       20              
  Lines        2477     2474       -3     
==========================================
- Hits         1876     1873       -3     
  Misses        601      601              

Impacted Files	Coverage Δ
lib/src/tasks/lexical_decision.dart	100.00% <100.00%> (ø)
lib/src/tasks/n_back.dart	100.00% <100.00%> (ø)

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