Description of proposed changes

Introduce new 'sequence_space' converter for transforming a Python list or tuple into a space separated string. Useful in cases for e.g. when multiple file inputs can be passed into some module.

For example:

@kwargs_to_string(files="sequence_space")
def module(*args, **kwargs):
    ...

>>> module(files=["data1.txt", "data2.txt"]
{'files': 'data1.txt data2.txt'}

Fixes #322

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.