adding new subpackages to begin refactoring core code
moving modules from tools to core
removing filter_key_type_mappings dictionary
replacing duplicate functions with imports from core
updating __init__ module imports
updating __init__ imports
adding new module of nanotube utility functions
updating docstring directives
moving nanogen subpackage modules to new subpackages
moving nanogen_gui to apps directory
renaming tools to utils
removing __init__ module
renaming structure_io to io
updating package imports
adding tests package to core package
moving _nanotube_funcs module to structures
removing _corefuncs module and useless check_type function
updating core package __init__ module imports
adding new _units module to core package for future dev
disabling core package module imports in __init__