Support for vector type

[Floriszenz]

It would be nice to support some sort of compound types, such as vector/tuple, which could be built from multiple columns. A possible interface is proposed in this issue comment.

Implementing vector types includes

• Adding a vector/tuple data type
• Adding option to merge single number columns into a vector column in load() method
• Adding option to split compound columns into single columns in load() method
• Trying to auto-merge columns if sensible, e.g. merging three number columns with names x, y, and z