Support Python3 & Python2

[FarnazH]

Description

Support Python3 (i.e., using iodata, grid, and gbasis) in addition to Python2 (i.e., using horton). So, chemtools can be imported/used with both Python 2 & 3 (some features would not be available with Python 3 yet, which results in an informative error). The tests can also be run with Python 2 & Python 3 (environments with proper dependencies installed) as well; some tests will be skipped depending on the Python version.

Checks

• [x] Each commit addresses one thing.
• [x] Each commit message is informative and is less than 50 characters.
• [ ] Commits that can be grouped together are squashed.
• [ ] Each unit of code added is tested.
• [ ] Each unit of code added is documented.
• [ ] Commits are rebased onto master.

Type of Changes

Please remove the lines that don't represent the type of your PR.

:sparkles: New Feature :art: Improve Format & Structure