Add day4 material

day4/HPC++_d4.pdf

day4/examples/CountingIterator.hh

day4/examples/README

day4/examples/jacobi/Makefile

day4/examples/jacobi/final.py

day4/examples/jacobi/jacobi_cxx17.cc

day4/examples/jacobi/jacobi_cxx20.cc

day4/examples/mandelbrot1.cc

day4/examples/mandelbrot_cpu.cc

day4/examples/mandelbrot_gpu.cu

day4/examples/matmul_trivial.cu

day4/examples/matmul_trivial2.cu

day4/examples/matmuld.cu

day4/examples/matmuls.cu

day4/examples/saxpy0.cu

day4/examples/thrust0.cu

day4/examples/thrust1.cu

day4/examples/transform_reduce.cu