Enforce positive amplitude for TabulatedFunctionModel

[jmborr]

Fixes #17

Code Review

• [ ] Is the code of an acceptable quality?
• [ ] Does the code conform to the [coding standards]?
• [ ] Are the unit tests small and test the class in isolation?
• [ ] If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• [ ] Do changes function as described? Add comments below that describe the tests performed?
• [ ] Do the changes handle unexpected situations, e.g. bad input?
• [ ] Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @idpflex/gatekeepers will take care of it.

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@1416475). Click here to learn what that means. The diff coverage is 25%.

@@            Coverage Diff            @@
##             master      #18   +/-   ##
=========================================
  Coverage          ?   83.89%           
=========================================
  Files             ?        7           
  Lines             ?      354           
  Branches          ?        0           
=========================================
  Hits              ?      297           
  Misses            ?       57           
  Partials          ?        0

Impacted Files	Coverage Δ
idpflex/properties.py	68.65% <0%> (ø)
idpflex/bayes.py	85.36% <100%> (ø)

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