Added internal normalization and associated tests for isotopes
Internal normalization does not make much sense for elements
Added extensive docs entry for how to use it
Added extensive background entry on why it is used / exists
test_main.py became too extensive, split it up into multiple files that take care of delta, bracket, ratio, and internal normalization subroutines. This adds better overview.
Add support for internal normalization. Closes #4
test_main.pybecame too extensive, split it up into multiple files that take care of delta, bracket, ratio, and internal normalization subroutines. This adds better overview.