Add generalized MultiModal data-loader ABC

[isaacmg]

We need a multi-modal data-loader that can load both time series data and recurring image data from the cloud. Moreover we need a data-loader that can work with CrossVIVIT and other TS/Image models.

• Research common image/time series models and how they load data. Write this up on Confluence.
• Create an ABC multi-modal loader that has methods that need to be implemented for either text, image or audio. This model should be a good base class for handling CSV or other files with images occurring at different intervals.
• Create documentation detailing how files should be laid out for multi-modal loader
• Create automated download of large files from GCP