add a document for code style

[marwahaha]

for example, do we want underscores? What casing?

[marwahaha]

consistent ordering from functions (should the ion go first)?

[marwahaha]

Electric field --> intensity?

[marwahaha]

• follow julia style guide -- but it's very slim
• "self-consistent"
• whitespace before and after
• docstrings should be julia style guide
• giving arguments to function -- have space after every comma
• spacing before and after binary operator
• indentations -- lines should be less than 70chr/line