make physicaliam return a Series if called with a Series

[wholmgren]

• Closes issue physicaliam returns np.array for pd.Series input #397
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests must pass on the TravisCI and Appveyor testing services.
• Code quality and style is sufficient. Passes git diff upstream/master -u -- "*.py" | flake8 --diff and/or landscape.io linting service.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.

@cwhanse let me know if you want me to make any more changes to the docstring. It seemed to me that much of the information was repetitive, so I deleted it.