ivis: dimensionality reduction in very large datasets using Siamese Networks

Highlights:

• Improved training speed for numpy arrray inputs thanks to a faster triplet generator.
• Batched retrieval capabilities that makes ivis much faster when training on out-of-memory data that is retrieved in parallel.
• Improved performance when using Ivis with precompute=False option by using multi-threading when retrieving batches of KNN on-demand.
• Added deprecation notices for minor upcoming changes to API for consistency and adherence to sklearn API.