Set to NaN if input datetime strings are not in ISO datetime format

[seisman]

Description of proposed changes

Set the value to NaN and raise an warning if the input string can't be parsed as ISO datetime format.

Fixes #3414

Reminders

• Correct base branch selected? master for new features, 6.1 for bug fixes
• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[weiji14]

Cool, thanks guys! I'll go and test it out after dinner, while recompiling gmt master from scratch 😀