Fix #10: LofarStMan is incompatible with CMake FetchContent

[aroffringa]

AI told me this should be fixed this way. This fixes #10 .

[Dantali0n]

AI is correct here, I did the same in #12 feel free to close 12 in favor or this or the other way around.

Below is an example of how to use this with Fetchcontent:

find_package(LofarStMan QUIET)

if(NOT LofarStMan_FOUND)
    set(ORIG_CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH})
    list(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")  #change for FindCasacore.cmake path
    FetchContent_Declare(
        LofarStMan
        GIT_REPOSITORY https://github.com/Dantali0n/LofarStMan.git
        GIT_TAG fix-fetchconten
    )
    FetchContent_MakeAvailable(LofarStMan)
    set(CMAKE_MODULE_PATH ${ORIG_CMAKE_MODULE_PATH})
endif()

[gmloose]

LGTM.