Add common alias cores (x) for grdimage and other multi-threaded modules

[weiji14]

Description of proposed changes

Used for setting the number of cores to be used in any OpenMP-enabled multi-threaded algorithms. E.g. for functions like grdimage, x2sys_solve (not yet available on PyGMT) and others. See also https://docs.generic-mapping-tools.org/6.1/cookbook/options.html#selecting-number-of-cpu-cores-the-x-option.

Note that the cores name follow upstream GMT's long option name convention at https://github.com/GenericMappingTools/gmt/blob/88421f9f8fe9759692302905268f4036b287adcb/src/gmt_init.c#L414

Related to #620.

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.