Expand ~ if used in path

[liyier90]

Use expanduser when ~ is detected in paths

[ongtw]

Pathlib has a Path.home().
Can use this when ~ is detected, instead of expanduser (which is from a diff os package).

[liyier90]

I was thinking of Path.expanduser() to just apply to config variables during the Node init, something like self.input_dir = Path(self.input_dir).expanduser().

The problem I'm having with this is deciding which paths we can apply this to, i.e., which path variables is allowed to accept full paths

[ongtw]

It is tricky to reconcile different OS/platforms due to different conventions, e.g. ~ / $HOME on Unix/Linux/MacOS, %homepath% / %userprofile% on Windows. Throw in the different shells, local vs network mounted user folder, etc., and the considerations become longer.

One option is to not support any of them, so that we:

1. don't appear to be platform biased by supporting either.
2. don't have to expand resources maintaining an exhaustive list of home variables.
3. avoid creating unintended bugs due to an unforeseen setup.

Stick with status quo where the user would have to specify the complete pathname: this is good since it is unambiguous.