Add basic project with gravity system test

.idea/ecsgame.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module classpath="CMake" type="CPP_MODULE" version="4" />

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="CMakeWorkspace" PROJECT_DIR="$PROJECT_DIR$" />
+  <component name="JavaScriptSettings">
+    <option name="languageLevel" value="ES6" />
+  </component>
+</project>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/ecsgame.iml" filepath="$PROJECT_DIR$/.idea/ecsgame.iml" />
+    </modules>
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.15)
+project(ecsgame)
+
+set(CMAKE_CXX_STANDARD 17)
+
+add_executable(ecsgame main.cpp)

main.cpp

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+#include <iostream>
+#include "ECS.h"
+
+using namespace ECS;
+
+struct Position
+{
+    Position(float x, float y, float z) : x(x), y(y), z(z) {}
+    Position() : x(0.f), y(0.f), z(0.f) {}
+
+    float x;
+    float y;
+    float z;
+};
+
+class GravitySystem : public EntitySystem
+{
+public:
+    GravitySystem(float amount)
+    {
+        gravityAmount = amount;
+    }
+
+    virtual ~GravitySystem() {}
+
+    virtual void tick(World* world, float deltaTime) override
+    {
+        world->each<Position>([&](Entity* ent, ComponentHandle<Position> position) {
+            position->y += gravityAmount * deltaTime;
+        });
+    }
+
+private:
+    float gravityAmount;
+};
+
+int main() {
+    World* world = World::createWorld();
+    world->registerSystem(new GravitySystem(-9.8f));
+
+    Entity* ent = world->create();
+    ent->assign<Position>(0.f, 0.f, 0.f); // assign() takes arguments and passes them to the constructor
+
+    world->tick(1.0f);
+
+    ComponentHandle<Position> pos = ent->get<Position>();
+
+    std::cout << "y position after tick: " << pos->y << std::endl;
+    return 0;
+}