fixes #276

[norweeg]

• removes unnecessary and silent type conversion of numeric node IDs
• add_edges using node IDs' original type no longer raises exception
• refactored coincidental inappropriate usage of assert for type-checking/control logic in add_node and add_nodes to raise appropriate exceptions instead

[norweeg]

general comment: I found the code rife with inappropriate and unsafe usage of assert as control logic/type checking. That should be completely refactored out because this library will fail spectacularly if run when __debug__ is False such as when running the interpreter with the -O flag or if the environment variable PYTHONOPTIMIZE is set

https://snyk.io/blog/the-dangers-of-assert-in-python/