Draft: core: Refactor SystemPath and use their functions in the codebase instead of system commands

Add Functions to SystemPath

• Add more functions to src/core/model/system-path.cc, including:
  • RemoveDirectories()
  • RemoveFile()
  • GetFileSize()
• Use those functions instead of directly invoking the Linux commands. This change makes the code more portable and robust.

SystemPath Refactor (Experimental)

• Refactor and simplify the implementation of the functions in SystemPath
• Replace system-path.{h,cc} with ns3-filesystem.h, which is a wrapper to the conditional compilation snippet used to decide which library to include: filesystem or experimental/filesystem.
• Use the filesystem library directly on the code, using the ns3-filesystem.h header file.
• Fix issue #781

NOTE: The animation-interface test is currently failing in this MR, due to issue #781.