Resolve "Failing tests in test_cluster_counts"

This fixes a bug in the implementation of ClusterCounts that makes a set of test cases in TestClusteCounts fails in MacOS with clang -O3.

In an earlier discussion this issue was related to a possible 'non-stable' ordering and therefore it was suggested to check if the tests were too restrictive. However after a close inspection I realized that not only the ordering in the cluster counts was different but actually the result was wrong.

Tracing back for the error source I found out that ClusterCounts.get_cluster_counts and ClusterCount.get_cluster_counts_info returns output with different ordering, which will cause some errors as the python implementation relies on the fact that both outputs have the same order.

Why does it fail only in MacOS? Apparently (as is mentioned here) assigning an unordered_map changes the internal order when using libc++ which is the default standard library on OSX. This does not happen with libstdc++ btw and might explain why the ordering is different when getClusterCount() is called.

How to solve this? After some IRL conversation with @angqvist it was decided to modify the actual implementation of ClusterCounts._count_clusters in the way that this only depends on the ClusterCounts.get_cluster_counts function. In addition, get_cluster_counts_info and set_cluster_counts_info wont be exposed in Pybind11 while some changes are introduced in ClusterCounts.get_cluster_counts in order to replace atomic numbers with atomic symbols.

Finally, _count_clusters output is sorted in order to avoid difference in the string representation when using clang.

Closes #270 (closed)