clang-tidy applied and CMake installation of the single header added (#287)

* Fixed headers and linter marks applied

Headers were wrong.
Applied some linter suggestions. There are still many left.

* CMake adjusted for single header

CMake can now install the single header.
Removed an unneeded header.

---------

Co-authored-by: Berscheid <1885260+pantor@users.noreply.github.com>

test/test-functions.cpp

@@ -1,5 +1,11 @@
 // Copyright (c) 2020 Pantor. All rights reserved.
 
+#include <doctest/doctest.h>
+#include <string>
+
+#include "inja/environment.hpp"
+#include "inja/inja.hpp"
+
 TEST_CASE("functions") {
   inja::Environment env;