Add extension icon and fix link in description file

[jcfr]

• Update icon https://github.com/VASST/VASSTAlgorithms/blob/master/VASSTAlgorithms.png
• Update CMakeLists.txt:

  VASSTAlgorithms/CMakeLists.txt

  Line 11 in b3b0db2

  set(EXTENSION_ICONURL "https://github.com/VASST/VASSTAlgorithms/blob/master/VASSTRegistration.png")

• Fix https://github.com/Slicer/ExtensionsIndex/blob/812eaa57786f94aa87798378d9de92bb3d437127/VASSTAlgorithms.s4ext#L30

[jcfr]

• Fix screenshot URLs:

  • VASSTAlgorithms/CMakeLists.txt

    Line 12 in 3f4b77a

    set(EXTENSION_SCREENSHOTURLS "http://www.example.com/Slicer/Extensions/VASSTRegistrationAlgo/Screenshots/1.png")

  • https://github.com/Slicer/ExtensionsIndex/blob/a564bbd1104bca304d062d9146112c77f0c07e17/VASSTAlgorithms.s4ext#L40

[adamrankin]

So, this gets weird. We don't have a screenshot as it is all code. Do you want a screenshot of a python interactor demonstrating usage?

[jcfr]

That would be great.