Add the dependency to the gym library

* Include a conda environment file
* To create a new environment execute:
  - conda env create -f environment.yml

environment.yml

+name: xgprl
+channels:
+ - defaults
+ - conda-forge
+dependencies:
+ - python==3.8
+ - cycler==0.10.0
+ - pandas==1.3.3
+ - matplotlib==3.4.3
+ - numpy==1.21.2
+ - pygraphviz==1.7
+ - pyparsing==2.4.7
+ - python-dateutil==2.8.2
+ - pyyaml==5.4.1 
+ - scikit-learn==0.24.2
+ - scipy==1.7.1
+ - six==1.16.0 
+ - threadpoolctl==2.2.0
+ - typing-extensions==3.10.0.2
+ - pip
+ - pip:
+   - deap==1.3.1
+   - joblib==1.0.1
+   - kiwisolver==1.3.2
+   - pillow==8.3.2
+   - psutil==5.8.0
+   - pytz==2021.1
+   - sklearn==0.0
+   - qdpy==0.1.2.1
+   - gym==0.20.0
+   - git+https://github.com/benelot/pybullet-gym
\ No newline at end of file

requirements.txt

@@ -59,3 +59,5 @@ threadpoolctl==2.2.0
     # via scikit-learn
 typing-extensions==3.10.0.2
     # via qdpy
+gym==0.20.0
+git+https://github.com/benelot/pybullet-gym	

requirements_with_pygraphviz.txt

@@ -61,3 +61,5 @@ threadpoolctl==2.2.0
     # via scikit-learn
 typing-extensions==3.10.0.2
     # via qdpy
+gym==0.20.0
+git+https://github.com/benelot/pybullet-gym