* Add support for Pascal Context 59 classes (#459)

* Create PascalContextDataset59 class in mmseg/datasets/pascal_context.py;

* Set reduce_zero_label=True for train_pipeline and PascalContextDataset59;

* Add some configs for Pascal-Context 59 classes training and testing;

* Try to solve the problem about "fence(IoU)=nan grass(IoU)=0";

* Continue(1): Try to solve the problem about "fence(IoU)=nan grass(IoU)=0";

* ignore files and folders named tempxxx;

* Continue(2): Try to solve the problem about "fence(IoU)=nan grass(IoU)=0";

* Modify the calculation of IoU;

* Modify the CLASSES order of PascalContextDataset;

* Add "fcn", "deeplabv3", "deeplabv3+", "pspnet" config file for model training based on PascalContextDataset59;

Add some ignore items in ".gitignore";

* fix the bug "test_cfg specified in both outer field and model field " of pspnet config file;

* * Clean unnecessary codes;

* Add weighs link, config link, log link and evaluation results about PascalContextDataset59 in README.md

* Add command line argument: "-p | --port", this arg can change the transmit port when you transmit data to distributed machine.

* * Remove rebundant config files;

* Remove "-p|--port" command argument;

Co-authored-by: Jiarui XU <xvjiarui0826@gmail.com>