Combine derived and heterogeneous meters

Currently, the multi meter arithmetic transformer can be used to combine only samples of simple and homogeneous meters.
Simple meters are meters whose samples are produced directly by sources, without transformations. An example is the *cpu* meter, produced by the *cpu* source.
Homogeneous meters are meters whose samples are produced by the same source. Examples are *disk.read.requests* and *disk.write.requests*, produced by the *disk* source.

With this blueprint we want to enable the capability to combine derived and heterogeneous meters.
A derived meter is a meter whose samples have been subjects to one or more transformations. An example is the *cpu_util* meter, that in libvirt is produced from samples of the *cpu* meter after having applied the *rate of change* transformer.
Heterogeneous meters are meters whose samples are produced by different sources. Examples are *cpu_util* and memory_usage*.

By enabling this capability we increase the flexibility of the multi meter arithmetic transformer.
For example we can define a new meter like the following:

cpu_memory_usage = 0.5*cpu_util/100 + 0.5*memory.usage/100