mpm2: find url given name

[mobeets]

Need a function in matlab to find the correct url to download given a name (e.g., "export_tikz") and any of the user's arguments regarding where to search first (i.e., github vs. matlab exchange), and a release_tag (github only). See find_package() in mpm.py for current functionality.

Two pieces to this:

• search github (find_github_repo() in find_package.py)
  • user has option to set release_tag
  • python code uses the github pip package, which probably doesn't exist in matlab?
• search Matlab Central (find_matlabcentral_repo() in find_package.py)

websave will probably be useful

This code should go in findUrl() in mpm2.m