Add functionality to strain data

A set of changes to improve our handling of strain data. The ultimate aim is to handle all strain data in one unified approach.

Tasks:

• Add time domain data and automatically get the frequency domain (partial fix for #121 (closed))
• Add methods for setting data from CSV files of time-domain strain [removed as this can be done easily with gwpy]
• Add units and explanations for the normalization
• Add set_from_gwpy method
• Encapsulate all other gwpy calls
• Add a create_psd method that turns the strain data into a power spectral density.
• Check documentation

@paul-lasky @colm.talbot @moritz.huebner @nikhil.sarin