Fix NumPy Serialization

[ax3l]

Fix #67

• add test with reproducer for MultipoleP
• add fix for MultipoleP
• develop a more generic serializer that can be used for any location where we have floats and lists

[ax3l]

cc @ruisardkj @adb-xkc85723 NumPy fix attempt :)

[EZoni]

First review batch to suggest using pytest's tmp_path fixture to avoid polluting and cleaning the root directory (no try/finally, no os.remove, etc.). I think we should do this in pre-existing tests as well, in a separate PR.