API for ClusterVector

[erikfrojdh]

Currently the ClusterVector [include/aare/ClusterVector.hpp] is templated on the data type and the coordinate type, but the cluster size in X and Y is set at runtime. I think we should gather some experience in how this works but be open to change the API to a also template on the cluster size.

Good:

• Avoids a combinatorial explosion of types (MxN) if we want to support any cluster size between 1 and 10
• Easier to return from ClusterFinder without having to also template the cluster finder on the size...

Not so good:

• Interface is not safe. Easy to mess up casting
• Missed optimizations?
• Awkward usage. Cannot just do push_back on a Cluster

Open questions:

• Is the API ok as it is?
• Is it worth templating the cluster size?