Add option to provide thirdparty dependencies externally

[Ylannl]

This adds a CMAKE option VAL3DITY_USE_INTERNAL_DEPS (default ON). When leaving this to the default (ON) it should build as before, ie. bundling the files in thirdparty as a val3dity_thirdparty lib and linking against that.

However, when set to OFF, it is expected that those 'thirdparty' dependencies (spdlog, nlohmann-json, tclap, pugixml) are available externally, eg. through vcpkg, and findable using CMAKE's find_package(). This is helpful in case val3dity is integrated as a library into another software that also depends those 'thirdparty' dependencies. I specifically had issues with spdlog, as this was publicly linked and it was a different version from what I used in my project, which can lead to undefined behaviour, unexpected crashes etc.

In addition

1. VAL3DITY_LIBRARY is now an option, so can be easily toggled when using ccmake or cmake-gui.
2. I restructered a bit the CMAKE file to make the above possible without making things too messy. Most notably I created an intermeditate val3dity_core object that contains most of the val3dity source code (all except main.cpp and val3dity.cpp). This object is reused both for the val3dity library and executable.
3. I made linking as much as possible PRIVATE to prevent pulling unwanted includes into projects that use val3dity as a library.
4. I had to change a few #include statements for nlohmann-json and pugixml to work in case they are externally provided. For the same reason I renamed the thirdparty/nlohmann folder.

[hugoledoux]

many cheers