Required dependencies, lazy imports

[adamjstewart]

Our number of dependencies is rapidly increasing. We should think about which of these dependencies are required (install_requires) vs. optional (extras_require). Here is a proposal:

1. Required: dependencies that are needed to do just about anything with TorchGeo
2. Optional: dependencies that are only needed for optional functionality, or for a small number of datasets

This gets a bit tricky, and is currently at odds with our dependency list. For example, rasterio is only used in RasterDataset, and fiona is only used in VectorDataset. While almost half of our datasets are RasterDataset, ony 1 is currently VectorDataset. Also, matplotlib is only needed to plot example samples.

Here is another proposal:

1. Required: any dependency that is used in regular usage
2. Optional: dependencies used only in rare cases (single dataset)

This may be a better default. Most users will likely run pip install torchgeo, which will install only the things in install_requires. We don't want a useless installation to be the default, and extras_require is off by default. Alternatively, we could do a better job of documenting the recommended way to install TorchGeo and specify that you may want pip install torchgeo[datasets,train] or something like that.

Another thing to consider is how to handle these optional imports. We can't put them at the module level (in the case of datasets) so we use lazy imports instead. We also may want to create a wrapper like importorraise (akin to pytest's importorskip) that prints a more useful error message upon ImportError. We could go even further and use lazy imports for almost all imports, not just optional ones. This will greatly speed up importing torchgeo.