move gnuplot function to displayer

now plot the figure in displayer with displayer->plot(mesh pointer, name)
modify Makefile and main.cpp to fit cuda require
Complete function to CUDA, though no solution
Complete function to CUDA

there are some problem with solution.
1. source position incorrect
2. not single side source
Merge branch 'CUDA' of github.com:lc85301/FDTD into CUDA

Conflicts:
	src/FDTD.cpp
	src/solver.cu