Figure.meca: Fix the bug when passing a dict of scalar values to the spec parameter

[seisman]

Description of proposed changes

Minimal example:

import pygmt

fig = pygmt.Figure()
fig.meca(
    spec=dict(strike=0, dip=90, rake=0, magnitude=5, longitude=0, latitude=5, depth=0),
    scale="2.5c",
    region=[-1, 1, 4, 6],
    projection="M14c",
    frame=2,
)
fig.savefig("meca.png")

Running the example above produces the following error messages:

Traceback (most recent call last):
  File "/Users/seisman/OSS/gmt/pygmt/workspace/test2.py", line 4, in <module>
    fig.meca(
  File "/Users/seisman/OSS/gmt/pygmt/pygmt/helpers/decorators.py", line 578, in new_module
    return module_func(*args, **kwargs)
  File "/Users/seisman/OSS/gmt/pygmt/pygmt/helpers/decorators.py", line 718, in new_module
    return module_func(*args, **kwargs)
  File "/Users/seisman/OSS/gmt/pygmt/pygmt/src/meca.py", line 286, in meca
    spec = pd.DataFrame(spec)
  File "/Users/seisman/opt/miniconda/lib/python3.9/site-packages/pandas/core/frame.py", line 636, in __init__
    mgr = dict_to_mgr(data, index, columns, dtype=dtype, copy=copy, typ=manager)
  File "/Users/seisman/opt/miniconda/lib/python3.9/site-packages/pandas/core/internals/construction.py", line 502, in dict_to_mgr
    return arrays_to_mgr(arrays, columns, index, dtype=dtype, typ=typ, consolidate=copy)
  File "/Users/seisman/opt/miniconda/lib/python3.9/site-packages/pandas/core/internals/construction.py", line 120, in arrays_to_mgr
    index = _extract_index(arrays)
  File "/Users/seisman/opt/miniconda/lib/python3.9/site-packages/pandas/core/internals/construction.py", line 664, in _extract_index
    raise ValueError("If using all scalar values, you must pass an index")
ValueError: If using all scalar values, you must pass an index

The error is raised when we try to convert a dict with scalar values to pandas.DataFrame. So, here is the real minimal example to reproduce the issue:

import pandas as pd
pd.DataFrame(dict(strike=0, dip=90, rake=0, magnitude=5, longitude=0, latitude=5, depth=0))

The solution here is to convert dict's values to ndarray before the converison.

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.

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[seisman]

Ping @GenericMappingTools/pygmt-maintainers for review.

[michaelgrund]

Looks good!

[weiji14]

Have you tested passing in the event_name name as a string too? Want to see if np.at_least1d handles it properly.

[seisman]

It works, because np.atleast_1d("abc") returns array(['abc'], dtype='<U3').

I've revised the test to also pass a scalar event_name argument.