Improve the readability of the grdview function

[seisman]

Description of proposed changes

When I worked on PR #750 based on the implemention of the grdview function,
I found that the grdview codes are a little difficult to understand.
The main reason is that contextlib.ExitStack() is new to me.
Another reason is that these two codes fname = stack.enter_context(file_context)
and arg_str = " ".join([fname, build_arg_string(kwargs)]) are
separated by the long if block.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[seisman]

@weiji14 Please see if you agree with these changes.

[weiji14]

Yep, looks good! Better to keep the variables fname closer to where it is used.