Improve upload and general data handling (e.g. via pandas)

[KMouratidis]

1. Correctly read various formats:

• [x] csv
• [x] xls / xlsx
• [x] json
• [x] feather
• [ ] html

2. Convert columns to appropriate data-types (possibly using heuristics). Focus on imporved handling of:

• [x] Dates, timestamps, etc
• [x] Categorical (Ordinal & Nominal)
• [ ] Missing values (interface with some sort of preprocessing pipeline? algorithm dependent?)
• [ ] Interface for custom objects (?)

3. Handling of files, storage, etc

• [x] Review use of Redis, discuss alternatives or improved usage, etc
• [x] Multiple file uploads
• [ ] Large file uploads (e.g. Resumable upload)