Add/edit cubids.py docstrings (#411)

* Add is_longitudinal attribute to the CuBIDS class

* fix lint issues

* still fixing lint issues

* Update cubids.py

* unset default value for is_longitudinal and add a method to infer is_longitudinal from data structure

* fix lint issues

* remove is_longitudinal from CuBIDS class docstring as Taylor suggested

* DOCSTRINGS

* lint

* fixed doctests

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Co-authored-by: Tien Tong <[email protected]>
Co-authored-by: Taylor Salo <[email protected]>
Co-authored-by: Tien Tong <[email protected]>