Remove Inline Output Mode; Automate Inlining in Composer

Closes #9 (closed)

• Remove inline output mode since it no longer makes sense
• Refactor the output_mode configuration option to the header_only boolean flag
• Refactor CMake function interface to take HEADER_ONLY option instead of OUTPUT_MODE argument
• Update composer to automatically generate inline definitions for functions, methods, and kernels, in header-only output mode
• Add a short section on header-only codegen to the docs

docs/source/usage/config_and_cli.md

@@ -42,14 +42,15 @@ The file extensions of the generated files can be modified through
 {any}`cfg.extensions.header <FileExtensions.header>`
 and {any}`cfg.extensions.impl <FileExtensions.impl>`;
 and the output directory of the code generator can be set through {any}`cfg.output_directory <SfgConfig.output_directory>`.
-Header-only code generation can be enabled using {any}`cfg.header_only <SfgConfig.header_only>`.
+The [header-only mode](#header_only_mode) can be enabled using {any}`cfg.header_only <SfgConfig.header_only>`.
 
 :::{danger}
 
-When running generator scripts through [CMake](#cmake_integration), you should *never* set the file extensions
-and the output directory in the inline configuration.
-Both are managed by the pystencils-sfg CMake module, and setting them manually inside the script will
-lead to an error.
+When running generator scripts through [CMake](#cmake_integration), the file extensions,
+output directory, and header-only mode settings will be managed fully by the pystencils-sfg
+CMake module and the (optional) project configuration module.
+They should therefore not be set in the inline configuration,
+as this will likely lead to errors being raised during code generation.
 :::
 
 ### Outer Namespace
@@ -89,6 +90,17 @@ with the `--help` flag:
 $ python kernels.py --help
 ```
 
+(header_only_mode)=
+## Header-Only Mode
+
+When the header-only output mode is enabled,
+the code generator will emit only a header file and no separate implementation file.
+In this case, the composer will automatically place all function, method,
+and kernel definitions in the header file.
+
+Header-only code generation can be enabled by setting the `--header-only` command-line flag
+or the {any}`SfgConfig.header_only` configuration option.
+
 (custom_cli_args)=
 ## Adding Custom Command-Line Options