Fix inconsistent argument names in `*load*` and `*save*` methods

[xroynard]

We should also check path argument names in all methods of Sample, Dataset and ProblemDefinition

[xroynard]

Current state:

• Dataset:
  • __init__ -> directory_path
  • save -> fname
  • _save_to_dir_ -> save_dir
  • load -> fname
  • load_from_file -> fname
  • load_from_dir -> dname
  • _load_from_dir_ -> save_dir
• Sample:
  • __init__ -> directory_path
  • save -> dir_path
  • load -> dir_path
  • load_from_dir -> dir_path
• ProblemDefinition:
  • __init__ -> directory_path
  • load -> save_dir
  • _load_from_dir_ -> save_dir
  • _save_from_dir_ -> save_dir

[xroynard]

maybe path would be the best name for these arguments, it works for directories and files

this would introduce an API retrocompatibility, check #97

currently save_dir is the most used, but it would not work for files, and maybe it should not be used for *load* methods