Allow an odd layer number / asymmetric stackups (lp:#1829379)

KiCad should allow specifying asymmetric and unusual stackups, meaning things like:

• Flex circuits with 1, 3, 5... layers
• metal core / insulated metal substrate stackups

Support needs to include at least the board stackup data storage and editor; 3D model generation (at least, thickness and color handling), and fabrication outputs.

Original report created by Hildo Guillardi Júnior (hildogjr)

Allow an odd layer number in Pcbnew. Currently it just allow even copper
layer quantity for create the pair logic.

This request/enhancement come up from question
https://answers.launchpad.net/kicad/+question/680850

Application: Pcbnew
Version: 6.0.0-unknown-a32bb4e~86~ubuntu16.04.1, release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.47.0 OpenSSL/1.0.2g zlib/1.2.8 libidn/1.32 librtmp/2.3
Platform: Linux 4.4.0-146-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.58.0
    OpenCASCADE Community Edition: 6.8.0
    Curl: 7.47.0
    Compiler: GCC 5.4.0 with C++ ABI 1009

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

Original tags: pcbnew