Fix repeated use of build_remapping with a patch

[sandorkertesz]

This PR fixes the issue when repeated use of remapping with a patch lost the original remapping. Now this works as expected and in the following code r1 and r2 define the same remapping (and patches).

from earthkit.data.core.order import build_remapping
remapping = {"type": "{type}{number}"}
patch = {"type": {"cf0": "x"}}
r1 = build_remapping(remapping, patch)
r2 = build_remapping(r1)

This PR also adds tests for using remapping and patches.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.96%. Comparing base (4973af9) to head (024f32b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #412      +/-   ##
===========================================
+ Coverage    89.92%   89.96%   +0.03%     
===========================================
  Files          120      121       +1     
  Lines         7764     7794      +30     
  Branches       675      679       +4     
===========================================
+ Hits          6982     7012      +30     
  Misses         651      651              
  Partials       131      131              

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