Add sequence_space converter in kwargs_to_string

[weiji14]

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.

[weiji14]

While working on grdview in #330, I've also been thinking about how to handle the -Ggrd_r -Ggrd_g -Ggrd_b case. The way I've implemented it in https://github.com/GenericMappingTools/pygmt/pull/330/files#diff-d304eb8534d09d80d3ec5bd2ba1f26b8R330-R336 actually allows drapegrid="red.nc -Ggreen.nc -Gblue.nc" since I didn't wrap it in a dummy_context but that's more a hack than an elegant solution...

Might open another PR on this so that users can do drapegrid=["red.nc", "green.nc", "blue.nc"] and the script will parse it to -Gred.nc -Ggreen.nc -Gblue.nc.

[seisman]

It should already works, as we already can have multiple -B options.

import pygmt
fig = pygmt.Figure()
fig.basemap(region="0/10/0/10", projection="X10c", frame=["xaf", "yaf", "+tTitle"])
fig.show()

[weiji14]

Wow, I never knew that worked! Must have missed that in one of the tutorials 😱

[seisman]

Looks good to me. Feel free to merge it @weiji14