Makes it possible to use for examples. Also add SingleCellExperiment and
Seurat dummy functions.

Needs to be exported for examples

* Remove temporary workaround

* use `inherits()` instead of `is()`

* Add HDF5AnnData and InMemoryAnnData to `read_h5ad()`

* fix incorrect merge

* try to fix error in R devel

* fix docs

* add terminal newline

* remove conditions

* undo previous commit

* support rhdf5 nullable vector and boolean enum implementation

- use `as.vector()` to reduce code duplication when reading
  nullable vectors
- closes #82

* implement and use read_h5ad_attributes

- 'quietly' to suppress warnings; set to TRUE when relevant warnings
  are handled by calling code

* use 'anndataR-category-unknown' as a class of warning

- versions of rhdf5 that don't support ENUM; more user-friendly message
- use to suppress irrelevant warnings in unit tests

* Tidy after merge and style

* More tidying...

* Roxygenise

* Fix namespace

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Co-authored-by: Martin Morgan <[email protected]>
Co-authored-by: Luke Zappia <[email protected]>