[cmake] cleaning and preparing release

[simogasp]

This PR does not affect the code, it's a cleaning of the current cmake build system and add some files for the upcoming first release.

Here is a summary of the major changes:

CMake

• raise minimum version to 3.12 (otherwise cuda 10 is not detected correctly)
• moved all the CMake support files and templates in the root cmake folder
• using GNUInstallDirs for installation directories (fix CMake: Should use GNUInstallDirs #48 )
• build SHARED libraries by default
• add a summary message with all the settings
• all the products of the building can be now found in a (system)-(architecture) folder of the building folder (e.g., Linux-x64 or Darwin-x64)
• some variables have been renamed to avoid conflicts if the project is used inside another CMake project
• removed old FindPopsift
• fixed include directories to include proper dirs using CMake generator expression
• some definitions were not handled through the config file, fixed

Code

• using c lib the c++ standard way eg <cstdio> instead of <stdio.h>
• added printing lib version in the apps
• USE_NVTX was not using POPSIFT_IS_DEFINED

Misc

• added changelog
• added code of conduct
• added contributing
• added clang-format file (same as AV)
• added templates for issues, PR and stale
• minor fixes in README

[griwodz]

I made a change to the README because PopSift can actually also use float greyscale images as input, and I specified that the image must be on the host side first (it would be attractive to allow images that are already on the GPU at some point).

[simogasp]

cool, thanks!

[griwodz]

I have tested and I'm satisfied with the changes.

[fabiencastan]

Great work.