Simpm

Fixed imports to make them faster for discrete event simulation.
Fixed few spellings and numbering of entities.

Updated spelling for environment.

Fixed import problems
Added change log

Discrete event simulation now has Docstrings.

What's Changed
* Changed pmpy package structure. PmPy now has a `simulation` subpackage that includes `discrete_event` and `distributions` modules. by NaimaSadeghi in bce1ea826a95285d817ffdab54bf6a71a4a870c3 and PedramElmi in https://github.com/Project-PmPy/pmpy/pull/4


**Full Changelog**: https://github.com/Project-PmPy/pmpy/compare/1.0.3...1.0.4

What's Changed
* Fixed pypi project description by PedramElmi in https://github.com/Project-PmPy/pmpy/pull/3


**Full Changelog**: https://github.com/Project-PmPy/pmpy/compare/1.0.2...1.0.3