ndnSIM

Previously, NFD repository was imported into ndnSIM and merged into NFD/ folder using git subtree command. Unfortunately, this organization is not ideal from maintainability perspective.

Alternative that simplifies maintenance is git submodule system. ndnSIM repository will be pointing to specific commit of the customized NFD and ndn-cxx repositories. When needed, this commit can be changed, including changes to rebased commits.

A small downside is a little complication of operations on repositories, as multiple repositories now involved.

Another part of this feature is to add custom form of ndn-cxx library as submodule. This way, ndnSIM no longer has indirect dependency on the library.