Loompy

We had acquired a dependency on `h5py>=2.10.0` when we introduced variable-length UTF8 string attributes (specifically, we used `h5py.string_dtype`). This was sad, because [scanpy](https://github.com/theislab/scanpy/blob/master/requirements.txt) has a dependency `h5py!=2.10.0` which makes loompy and scanpy strictly incompatible. However, it turns out there's a way to describe the same thing that works in older versions of h5py (specifically, using `special_dtype(vlen=str)`). This release uses the old way, and removes the dependency on h5py 2.10.0.

Validator now automatically validates against the spec version that the file itself declares (rather than a spec version requested by the caller). This can be overridden by specifying a specific version for validation.

The validator was upgraded to conform to the 3.0.0 spec.

Fixes a couple of minor bugs

* Missing dependency added: numpy_groupies
* Accidental Python 3.7 dependency removed

Updates the Loom v3.0.0 file format specification

Minor release mainly with updated docs and tutorials for `loompy fromfq` command-line tool.