fix: module not found in __init__.py

[zayn7lie]

For Linux, without . in __init__.py installed in \usr\lib will cause:

Traceback (most recent call last):
  File "/usr/lib/python/site-packages/HierMat/__init__.py", line 1, in <module>
    from block_cluster_tree import BlockClusterTree, build_block_cluster_tree
ModuleNotFoundError: No module named 'block_cluster_tree'