Changes that could easily be extracted from #2027.

• Tell clang-format how to interpret PyObject_HEAD
• Drop isoc99.{h,cpp} and nrn_isdouble function
• Update submodule to include Allow Custom build type iv#45 so one can build with -DCMAKE_BUILD_TYPE=Custom and -DNRN_ENABLE_INTERVIEWS=ON.
• Add missing include in src/ivoc/ocnotify.h
• Drop a lot of unused code in cxprop.cpp -- note that the 6th line of nrn_cache_prop_realloc was a return statement, and has been for some time.