pre-release commit for v0.5.0

[wsavran]

• add _version.py
• update changelog to markdown for github formatting
• update codemeta.json with version information
• add sphinx to requirements in setup.py
• add instructions to documentation about issuing releases

[wsavran]

@pjmaechling this pr is pre-release that contains small changes that should be merged before we issue a new release. this bumps the version and updates various files in the package. due to the large number of changes since v0.4.1 i have decided to issue a major release for v0.5.0. i've added a development roadmap for v0.6.0 in the documentation.

you can see a draft of the release here that contains the CHANGELOG for v0.5.0.

once this release is issued (tagged on github) zenodo will generate a DOI for the release and I'll update the readme.md file and add a new badge. also, I'll add the other badges that you suggested.

[codecov-commenter]

Codecov Report

Merging #149 (58ab70e) into master (9e53c0a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   56.48%   56.50%   +0.02%     
==========================================
  Files          19       20       +1     
  Lines        3233     3235       +2     
  Branches      479      479              
==========================================
+ Hits         1826     1828       +2     
  Misses       1288     1288              
  Partials      119      119              

Impacted Files	Coverage Δ
csep/__init__.py	32.77% <100.00%> (+0.56%)	⬆️
csep/_version.py	100.00% <100.00%> (ø)

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