CMake Interaction Update

Closes #6 (closed) , #8 (closed) .

• Allow the user to specify the Python interpreter to be used by the pystencils-sfg CMake module via a hint variable to the find module
• Allow the user to pass custom command line arguments to generator scripts through CMake
• Move the default output path for generated files to ${CMAKE_CURRENT_BINARY_DIR}/_gen/<target>/gen
• Allow the user to customize the output directory

docs/source/usage/project_integration.md

@@ -122,6 +122,7 @@ pystencilssfg_generate_target_sources( <target>
     [FILE_EXTENSIONS <header-extension> <impl-extension>]
     [OUTPUT_MODE <standalone|inline|header-only>]
     [CONFIG_MODULE <path-to-config-module.py>]
+    [OUTPUT_DIRECTORY <output-directory>]
 )
 ```
 
@@ -138,8 +139,11 @@ The function takes the following options:
  - `CONFIG_MODULE`: Set the configuration module for all scripts registered with this call.
    If set, this overrides the value of `PystencilsSfg_CONFIG_MODULE`
    in the current scope (see [](#cmake_set_config_module))
+ - `OUTPUT_DIRECTORY`: Custom output directory for generated files. If `OUTPUT_DIRECTORY` is a relative path,
+   it will be interpreted relative to the current build directory.
 
-Any C++ header files generated by the above call can be included in any files belonging to `target` via:
+If `OUTPUT_DIRECTORY` is *not* specified, any C++ header files generated by the above call
+can be included in any files belonging to `target` via:
 
 ```C++
 #include "gen/<file1.hpp>"
@@ -147,12 +151,12 @@ Any C++ header files generated by the above call can be included in any files be
 /* ... */
 ```
 
-### Changing the Output Directory
+:::{attention}
+If you change the code generator output directory using the `OUTPUT_DIRECTORY` argument,
+you are yourself responsible for placing that directory--or any of its parents--on the
+include path of your target.
+:::
 
-The CMake function listed above will create a subdirectory `_gen/<target>` at the current point in
-the build tree (i.e. [`CMAKE_CURRENT_BINARY_DIR`](https://cmake.org/cmake/help/latest/variable/CMAKE_CURRENT_BINARY_DIR.html)).
-This directory is placed on the include path of `<target>`, and the generated files will be written to `_gen/<target>/gen/`
-such that they can be included with the `gen` prefix.
 
 (cmake_set_config_module)=
 ### Set a Configuration Module