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