Fix prior scales

[yannikschaelte]

• fix get_prior_from_df returning wrong scales if using default bounds
• fix get_prior_from_df handling missing entries badly
• add tests

[codecov-io]

Codecov Report

Merging #286 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop     #286   +/-   ##
========================================
  Coverage    76.99%   76.99%           
========================================
  Files           22       22           
  Lines         1804     1804           
  Branches       390      390           
========================================
  Hits          1389     1389           
  Misses         319      319           
  Partials        96       96

Impacted Files	Coverage Δ
petab/lint.py	74.43% <ø> (ø)	⬆️
petab/core.py	84.87% <100%> (ø)	⬆️

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