Fix Read{,List}[] with TokenWord and improve ...

[rocky]

• Read{,List}[] with TokenWord needs to return two tokens sometimes
• Adjust error message the right value of Read or ReadList is used.
• simplify logic in eval/files_io/read.py
• Add data_dir variable that can be used in testing.

[rocky]

There will be at least two more rounds of PRs, but this is already getting large.

One round involves splitting out Reading & Writing Files from mathics.builtins.file_io.files . And with this, the corresponding tests in test_files.py will be moved. And test_read will be renamed probably to readwrite with mathics.builtins.file_io.readwrite getting created. So mathics.eval.files_io.read will also get renamed.

In this test for this PR, I suggest a better way of accessing test data than using StringToStream, and this probably could simplify other existing tests as well.

There will be another round to increase test coverage. But do it in a way that is not as messy as the pytest and doctest code we have right now.

I am always amazed at the pile of mess we still are digging out of.

A second round will be

The I/O routines are just such a big mess

[mmatera]

LGTM