Selector-based decoding mockup; only handles a couple attributes
updated h5decode.R to use Selector
Create an AtomicSelector for an integer vector with and without attrs works

- In the process of moving from Selector to RecursiveSelector, Atomic
  Selector, and ListLikeSelector
- Implemented show methods for RSel, ASel, LLSel, and h5id class
Subsetting a named vector works
Subsetting an attribute works, tested

- changed test to just subset the attribute
- all tests in test_attributes.R pass
creating RSelector for unnamed list works, tested

- tests later in file not implemented
create RSelector for unnamed list without data works, tested
RSelector for named list works, tested

- next is materializing whole named list
materialize whole unnamed vector from RSelector works, tested
Materialize whole named vector with data works, tested

- PROBLEM: need to keep track in RecursiveSelector *which* h5data
  elements have been selected, not just the length as I was planning
subset unnamed list works, tested
Subsetting named list works, tested

- added selection_indices member to RecursiveSelector
Create class RectSelector works, tested

- Added a class, 'RectClass', for representing rectangular VECSXP
  objects
materializing a factor works tested; Implicit now decodes by reading all
materialize data.frame with factor column works, tested
subsetting data.frame Selector works, tested
Materialize a whole S4 object from class defined in a package works, tested
Materialize whole IRanges works, tested
Add utility functions to help navigate encoded objs
separate testing of h5codeutils