Importing evo plotting tools results in headless CI failing

[YoshuaNava]

Description:
I implemented a thin wrapper over evo, along with unit tests which run on a CI server.

Whenever I try to run the tests locally, everything goes well. When I do so on the CI, I get the following exception:

(see Console Output)

This error happens because the CI is running everything in headless mode. To overcome it I tried setting the matplotlib backend to pdf by default

In my opinion the source of the issue lies here: https://github.com/MichaelGrupp/evo/blob/master/evo/tools/plot.py#L32
Where the matplotlib backend is set silently by evo. The backend defaults to PyQt5, which doesn't run in headless mode, and an exception is thrown.

I would suggest that the default backend is pdf instead: https://matplotlib.org/3.3.1/api/backend_pdf_api.html

Console output:

`ImportError: Cannot load backend 'Qt5Agg' which requires the 'qt5' interactive framework, as 'headless' is currently running`

Additional files:
Please attach all the files needed to reproduce the error.

Please give also the following information:

• evo version number shown by evo pkg --version: v1.11.0
• Python version shown by evo pkg --pyversion: 3.8.5
• operating system and version (e.g. Ubuntu 16.04 or Windows 10): Ubuntu 20.4
• did you change the source code? (yes / no): no
• output of evo_config show --brief --no_color:

{
    "console_logging_format": "%(message)s",
    "euler_angle_sequence": "sxyz",
    "global_logfile_enabled": false,
    "plot_axis_marker_scale": 0.0,
    "plot_backend": "Qt5Agg",
    "plot_export_format": "pdf",
    "plot_figsize": [
        6,
        6
    ],
    "plot_fontfamily": "sans-serif",
    "plot_fontscale": 1.0,
    "plot_invert_xaxis": false,
    "plot_invert_yaxis": false,
    "plot_linewidth": 1.5,
    "plot_mode_default": "xyz",
    "plot_multi_cmap": "none",
    "plot_pose_correspondences": false,
    "plot_pose_correspondences_linestyle": "dotted",
    "plot_reference_alpha": 0.5,
    "plot_reference_color": "black",
    "plot_reference_linestyle": "--",
    "plot_seaborn_palette": "deep6",
    "plot_seaborn_style": "darkgrid",
    "plot_split": false,
    "plot_statistics": [
        "rmse",
        "median",
        "mean",
        "std",
        "min",
        "max"
    ],
    "plot_texsystem": "pdflatex",
    "plot_trajectory_alpha": 0.75,
    "plot_trajectory_cmap": "jet",
    "plot_trajectory_linestyle": "-",
    "plot_usetex": false,
    "plot_xyz_realistic": true,
    "ros_map_alpha_value": 1.0,
    "ros_map_unknown_cell_value": 205,
    "save_traj_in_zip": false,
    "table_export_data": "stats",
    "table_export_format": "csv",
    "table_export_transpose": true,
    "tf_cache_lookup_frequency": 10,
    "tf_cache_max_time": 10000.0
}

[MichaelGrupp]

Thanks for reporting this. I have always set it manually in CI, but it's better to have a working default.

The latest release now uses Agg in headless environments after installation or after evo_config reset plot_backend. This fixes the crash. Writing a PDF later then switches to the pdf renderer implicitly for the file export as far as I can tell.

fbb6541