Replace pragma once with explicit header guards

[joka921]

While being practical when writing, #pragma once has several downsides.

1. It is not standardized.
2. It doesn't work when feeding header files to libclang-based tools.
3. It can break in very subtle ways in complex build environments.

This PR replaces all occurences of #pragma once by explicit header guards. The header guard macros contain the project name and the full path to the file and the filename, so they are very robust against collisions (e.g. QLEVER_SRC_ENGINE_OPERATION_H for the file src/engine/Operation.h.
The replacement has been performed by a Python script which is also checked in as part of this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.66%. Comparing base (0a47174) to head (062cde0).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1908   +/-   ##
=======================================
  Coverage   90.66%   90.66%           
=======================================
  Files         411      411           
  Lines       40111    40113    +2     
  Branches     4539     4539           
=======================================
+ Hits        36366    36368    +2     
- Misses       2398     2400    +2     
+ Partials     1347     1345    -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[sparql-conformance]

Conformance check passed ✅

No test result changes.

Details: https://qlever.cs.uni-freiburg.de/sparql-conformance-ui?cur=062cde0e7a4feba9f62ddc93bc38e9b8953b94a2&prev=0a47174744348e73c19d9c12232d46e012bdab8d

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud