Interpolator example is not working with lua5.2 nor lua5.3

[d-a-v]

Similar to issue 49 whith lua5.2:

On Ubuntu14.04, with liblua5.2-dev installed,

~/dev/S4/examples/interpolator$ ../../build/S4 interpolator.lua 
interpolator.lua:9: attempt to index global 'interpolator' (a userdata value)
stack traceback:
        interpolator.lua:9: in main chunk
~/dev/S4/examples/interpolator$ ldd ../../build/S4
        linux-vdso.so.1 =>  (0x00007ffe5a7fa000)
        liblua5.2.so.0 => /usr/lib/x86_64-linux-gnu/liblua5.2.so.0 (0x00007f46f4d72000)
        libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f46f4a5f000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f46f4758000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f46f4390000)
        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f46f418c000)
        /lib64/ld-linux-x86-64.so.2 (0x000055eb0fbf6000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f46f3f74000)

with exactly the same error on ubuntu 17.04 and lua5.3.

[d-a-v]

this pull request restores linear interpolator. a working example is also added. the spline interpolator needs further debugging.