All changes to allow SEDML to be used as submodule.

[luciansmith]

This version leaves in the 'submodules' directory, which I'll then delete from my own version instead. All told, the changes are:

• Remove 'dist' target.
• Don't check for zlib.h
• New advanced user options to turn off searching for libSBML and libnuml.
• Remove unused CMakeLists.txt file
• Turn on EXTRA_INCLUDE_DIRS in actually-used CMakeLists.txt file.