Identify bndbox coordinates by tag

[pixx1]

Made s small change in xml_to_csv.py
Use the XML tags to identify "ymin, xmin, ymax, xmax", and not only by position.

I made this, because I ran in some trouble. Had XML files with another order of the box coordinates.

[ghost]

Would love it if this one gets merged, ran into this issue as well, took a few hours to hunt it down, mostly because of the vague error message:

assertion failed: [maximum box coordinate value is larger than 1.100000: ] [1.18155193]

[Petros626]

@pixx1 can you say which order? I use a label tool, which generates me PASCAL VOC Format in XML files, but there is more information than for example using this tool (https://github.com/tzutalin/labelImg). Is this a problem for the script?