update precision arguments

[justusschock]

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Changes precision argument to only support lightning 2.0s precision='16-mixed'

Fixes #8

PR review

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[codecov]

Codecov Report

Merging #9 (b7d3229) into main (0103143) will not change coverage.
The diff coverage is 80%.

Additional details and impacted files

@@        Coverage Diff         @@
##           main    #9   +/-   ##
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  Coverage    35%   35%           
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  Files         4     4           
  Lines       274   274           
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  Hits         97    97           
  Misses      177   177