Simplify some parts of code

[anilbey]

Changes

• use itertools.product to eliminate nested loops
• remove reimport of a library
• replace index based for loop with enumerate
• and some smaller changes

[codecov[bot]]

Codecov Report

Merging #244 (80d2ed5) into master (0aee142) will decrease coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   84.12%   84.04%   -0.09%     
==========================================
  Files          22       22              
  Lines        1153     1147       -6     
==========================================
- Hits          970      964       -6     
  Misses        183      183              

Impacted Files	Coverage Δ
bluepymm/select_combos/reporting.py	98.66% <ø> (-0.01%)	:arrow_down:
bluepymm/select_combos/table_processing.py	85.91% <ø> (ø)
bluepymm/prepare_combos/parse_files.py	62.26% <100.00%> (+0.35%)	:arrow_up:
bluepymm/run_combos/calculate_scores.py	78.90% <100.00%> (-0.64%)	:arrow_down:
bluepymm/select_combos/megate_output.py	100.00% <100.00%> (ø)

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