add functionality to install needed header files

[vancraar]

Addressing the missing installation of header files mentioned in #140. @freifrauvonbleifrei Which headers should be installed and are they dependent on a certain folder structure, or can they simply all be located together in the include folder?

[freifrauvonbleifrei]

so far, we've been using them with the subfolder paths (#include "task/Task.hpp" etc.), but maybe it would be clearer for library users to use something like #include "discotec/task/Task.hpp" to avoid ambiguity in case someone has a similar/same named header -- what do you think @vancraar ?

[freifrauvonbleifrei]

(maybe we could skip the subpaths and go for #include "discotec/Task.hpp"?)

[freifrauvonbleifrei]

also I would be in favor of a CMake namespace, but this here appears to be insufficient (added in src/CMakeLists.txt):

add_library(discotecTargets INTERFACE)
add_library(discotec::discotec ALIAS discotec)

install(EXPORT discotecTargets
  FILE discotec.cmake
  NAMESPACE discotec::
  DESTINATION lib/cmake/discotec
)

@vancraar do you know how to do it?

[freifrauvonbleifrei]

@vancraar am I right to assume that we would have to introduce a src/discotec subfolder to have the includes work as discotec/task/Task.hpp for both the examples in-code (with add_subdirectory) and for code that uses the installed lib (with find_package)?