POC + WIP: Support multiprocessing Version 1

[seisman]

Description of proposed changes

This PR aims to address our oldest issue #217. See #3398 for the technical notes about GMT CLI sessions and GMT C API sessions.

Test scripts

1. The widely-used series version:

import pygmt
fig = pygmt.Figure()
fig.basemap(region=[0, 10, 0, 10], projection="X4c/4c", frame=True)
fig.show()

2. The parallel version:

import pygmt
import multiprocessing as mp
import os

def gmt_fun(n):
    fig = pygmt.Figure()
    fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=["a", f"WSen+t{n}"])
    fig.savefig("{}.pdf".format(n))

with mp.Pool(2) as p:
    a = p.map(gmt_fun,[x for x in range(0, 2)])

3. The series version but explicitly calls begin/end to manage sessions.

import pygmt
from pygmt.session_management import begin, end
begin()
fig = pygmt.Figure()
fig.basemap(region=[0, 10, 0, 10], projection="X4c/4c", frame=True)
fig.show()
end()

4. The paralle version but explicitly calls begin/end to manage sessions

import pygmt
from pygmt.session_management import begin, end
import multiprocessing as mp
import os

def gmt_fun(n):
    begin()
    fig = pygmt.Figure()
    fig.basemap(region=[0, 10, 0, 10], projection="X10c", frame=["a", f"WSen+t{n}"])
    fig.savefig("{}.pdf".format(n))
    end()

with mp.Pool(2) as p:
    a = p.map(gmt_fun,[x for x in range(0, 2)])

Fixes #217.

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash command is:

• /format: automatically format and lint the code