Refactor SIR <-> AST interdependencies

[muellch]

Technical Description

Gets rid of the SIR<-> AST circular dependencies resolving https://github.com/MeteoSwiss-APN/dawn/issues/319 and partially resolving https://github.com/MeteoSwiss-APN/dawn/issues/320.

The most important change is the separation of anything related to class sir::VerticalRegion, including class ast::VerticalRegionDeclStmt, into separate files VerticalRegion.h and VerticalRegion.cpp. In these files class sir::VerticalRegion remains defined as part of the sri namespace, while ast::VerticalRegionDeclStmt remains defined as part of the namespace ast.

Other noteworthy changes:

• class sri::Value, class sri::Interval and class sri::Attr have all been extracted into according header and .cpp files and moved to the namespacd ast.
• All sir and iir aliased of ast classes have been removed.
• A lot of #include directives have been cleaned up throughout the project (Thank you CLion).

Future discussions and TODOs:

• One part of https://github.com/MeteoSwiss-APN/dawn/issues/320 remains open: https://github.com/MeteoSwiss-APN/dawn/blob/10d08b48b78133464485f119372b07f6812ca3f1/dawn/src/dawn/AST/ASTStmt.h#L504-L513
• Should other classes in the sri namespace, such as class sri::StencilFunction or class sri::Field also be extracted into separate files and moved into the ast namespace?

[muellch]

launch jenkins

[muellch]

launch jenkins

[Stagno]

@muellch about the two points you mention in the description, I'd say yes, they could be done as followups to this PR if you still think AST refactorings are still worth your time.

[muellch]

launch jenkins

[Stagno]

@muellch Squash and merge at your will, as PR is approved and build is green