find root, min, max of func: update doc string and refactor kwargs check

[rioj7]

doc strings of root_scalar and maximize_scalar contained errors

refactored the check of the kwargs if it contains parameter bracket or bounds. minimize_scalar accepts both depending on method, and method has default value depending on which parameter is present

Why is there a modsim.py in the root of the repo? The chapterX.py files can also work with the modsim module directory.

[AllenDowney]

Thank you -- this looks good. I need to run the notebooks as tests, but will merge as soon as I can.

[AllenDowney]

Thank you!