Added UnknownUnitError

[jesper-friis]

Description:

Added UnknownUnitError.

Motivation: Using a unit that is not in the Pint unit registry results in a completely ununderstandable error message. The idea is to issue understandable UnknownUnitError's whenever an unknown unit is used.

Currently this error is only imported by DLite, but the intention is to used in tripper as well.

Type of change:

• Bug fix.
• New feature.
• Documentation update.
• Testing.

Checklist for the reviewer:

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fdbbd06) 75.08% compared to head (7477184) 75.10%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   75.08%   75.10%   +0.01%     
==========================================
  Files          17       17              
  Lines        1405     1406       +1     
==========================================
+ Hits         1055     1056       +1     
  Misses        350      350              

Files	Coverage Δ
tripper/mappings/mappings.py	66.39% <100.00%> (+0.09%)	⬆️

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