Update MHKiT-Python installation instructions

[simmsa]

Summary of changes:

• Recommends Conda as the preferred installer
• Adds details for installing Conda
• Adds command to download environment.yml from MHKiT-Python master
  • curl -o mhkit_environment.yml https://raw.githubusercontent.com/MHKiT-Software/MHKiT Python/master/environment.yml
  • This is a quick option for users to download the most current environment.yml
  • This worked for me using the anaconda terminal on Windows, but there are probably edge cases where this will not work
  • Added a link to the file if using curl doesn't work
• Adds install command using conda-forge, fixing #71
• Changes python test library to pytest, fixing #72

[simmsa]

@ssolson and @akeeste do either of you have any comments on this PR?