Added POMDP-value-iteration (#929)

* Added POMDP value iteration

* Added plot_pomdp_utility function

* Added tests for pomdp-value-iteration

* Updated README.md

* Fixed notebook import

* Changed colors

* Added notebook sections for POMDP and pomdp_value_iteration

* Fixed notebook parsing error

* Replace pomdp.ipynb

* Updated README.md

* Fixed line endings

* Fixed line endings

* Fixed line endings

* Fixed line endings

* Removed numpy dependency

* Added docstrings

* Fix tests

* Added a test for pomdp_value_iteration

* Remove numpy dependencies from mdp.ipynb

* Added POMDP to mdp_apps.ipynb