allow zero bounds for log parameters

[FFroehlich]

if there is no finiteness check for upper bound, I don't see why we should prevent lower bound 0 for log scale parameter, it convenient for specifying unbounded parameters after all.

[codecov-commenter]

Codecov Report

Merging #83 (4a18854) into develop (0a2de3b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop      #83   +/-   ##
========================================
  Coverage    79.59%   79.59%           
========================================
  Files           26       26           
  Lines         2940     2940           
  Branches       695      695           
========================================
  Hits          2340     2340           
  Misses         423      423           
  Partials       177      177           

Impacted Files	Coverage Δ
petab/lint.py	77.10% <100.00%> (ø)

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[dweindl]

Agreed