Relax importer for Pascal VOC dataset (seach in subdirectories)

[nmanovic]

In some cases developers don't want to specify the exact path to Pascal VOC.
Now you have to specify VOCtrainval_11-May-2012/VOCdevkit/VOC2012/. After the
patch it will be possible to specify VOCtrainval_11-May-2012/.

Summary

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[zhiltsov-max]

Looks good, however, now the structure:

a/b/c/ImageSets/Main/
a/ImageSets/Segmentation/

Is a valid dataset. But I don't think it is a real problem.

[nmanovic]

Looks good, however, now the structure:

a/b/c/ImageSets/Main/
a/ImageSets/Segmentation/

Is a valid dataset. But I don't think it is a real problem.

I agree that it isn't good. I have fixed the case.