Add further Dimensionality Reduction Algorithms

[Cryaaa]

Some more dimensionality reduction algorithms might be interesting for exploring other visualisations:

• [ ] Diffusion maps: might be interesting since used in other developmental biology projects. Implementation here with MIT license
• [x] ISOMAP and MDS: both in sklearn: easy implementation
• [ ] SPRING plots (come from scSeq analysis and are more suited for continuums of expression states)
• [ ] others? I'm open to suggestions here

[lazigu]

Added ISOMAP and MDS in #187