Fix the maximum allowed time span for F-net

[seisman]

Description of proposed changes

See #42 for the issue report.

The limit of Fnet data file size is 55 MB.

Closes #42.

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.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.