ENH: allow direct dataset instanciation using the user path '~' glyph, consistently with yt.load

[neutrinoceros]

PR Summary

In some occasion, it is useful (sometimes, mandatory) to load an exact dataset class explicitly. This happens with Boxlib-based datasets which are not easy to distinguish automatically with yt.load. It is also useful when developing a frontend.
Now, yt.load supports automatic expansion of ~/ via os.path.expanduser, however currently, this is not done in the Dataset classes themselves, resulting in an inconsistency where the first test pass, but not the second:

import yt
from yt.frontends.enzo.api import EnzoDataset

fn = "~/dev/yt-project/test-data-dir/IsolatedGalaxy/galaxy0030/galaxy0030"

def test_load():
    ds = yt.load(fn)

def test_api():
    ds = EnzoDataset(fn)

this fixes that by moving the os.path.expanduser call to Dataset.__init__

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[matthewturk]

modulo the one question LGTM