Replace all tabs with spaces in the org.lflang.diagrams package

[a-sr]

As suggested in #1040 this eliminates the use of tabs in the diagrams package.

[lhstrh]

This looks fine to me, but there appears to be a persistent Oomph-related build problem that I don't quite understand.

[lhstrh]

This problem now also breaks master:

Error:  Failed to resolve target definition file:/home/runner/work/lingua-franca/lingua-franca/org.lflang.targetplatform/org.lflang.targetplatform.target: Could not find "org.eclipse.xpand.feature.group/0.0.0" in the repositories of the current location -> [Help 1]
Error:  
Error:  To see the full stack trace of the errors, re-run Maven with the -e switch.
Error:  Re-run Maven using the -X switch to enable full debug logging.
Error:  
Error:  For more information about the errors and possible solutions, please read the following articles:
Error:  [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MavenExecutionException
[INFO] Scanning for projects...
[INFO] Computing target platform for MavenProject: org.lflang:org.lflang:0.1.0-SNAPSHOT @ /home/runner/work/lingua-franca/lingua-franca/org.lflang/pom.xml
[INFO] Resolving target definition file:/home/runner/work/lingua-franca/lingua-franca/org.lflang.targetplatform/org.lflang.targetplatform.target for environments=[win32/win32/x86_64, linux/gtk/x86_64, macosx/cocoa/x86_64], include source mode=honor, execution environment=StandardEEResolutionHints [executionEnvironment=OSGi profile 'JavaSE-11' { source level: 11, target level: 11}], remote p2 repository options=org.eclipse.tycho.p2.remote.RemoteAgent@632dc41...
Error:  Failed to resolve target definition file:/home/runner/work/lingua-franca/lingua-franca/org.lflang.targetplatform/org.lflang.targetplatform.target: Could not find "org.eclipse.xpand.feature.group/0.0.0" in the repositories of the current location -> [Help 1]
Error:  
Error:  To see the full stack trace of the errors, re-run Maven with the -e switch.
Error:  Re-run Maven using the -X switch to enable full debug logging.
Error:  
Error:  For more information about the errors and possible solutions, please read the following articles:
Error:  [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MavenExecutionException
Error: Process completed with exit code 1.