particle/ramp: new particle, selects one particle from a list

The ramp particle is configured with a tag name and a list of
particles.

At instantiation time, a single particle is selected from the list,
depending on the tag value and it's minimum and maximum values.

I.e. this particle acts kind of like a progress bar.

particles/ramp.h

@@ -0,0 +1,5 @@
+#pragma once
+#include "../particle.h"
+
+struct particle *particle_ramp_new(
+    const char *tag, struct particle *particles[], size_t count);