add check of f0 sum, norm if >1 #45

[sschmidt23]

Problem & Solution Description (including issue #)

Addresses the problem in issue #45 where the fractions f0 can sum to more than 1, which will force the last type to go negative at the m0 value and nearby values. I added an if statement and division to set to 1.0 if >1, which fixes the issue in tests that I ran. I'm not adding a unit test because the nelder-mead minimizer is fickle and it's sometimes hard to trigger, plus I have tested it in notebooks and it works as expected.

Code Quality

• My code follows the code style of this project
• I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings