Updated Estimator interface (#51)

* Filled out the Summary, Motivation, and User Benefit sections

* Filled out Alternative Approaches section

* Add Tasks section

* Add to Future Extensions section

* Add more minor edits

* Add Sections (#4)

* add to Future Extensions (re Task in BasePrimitive)

* Array broadcasting figure, example (#5)

* Add section references (#6)

* Add "BindingsArray generalizations" subsection

* Add subsection titles

* Add more BindingsArray examples

* Wording change

* Move the example up (no significant changes)

* Add detail to migration plan (#7)

* Light editing of first few sections.

* Move all of the class ideas into Detailed Design

* Edits in the detailed design subsections

* Edits to migration section

* Flesh out examples more

* update date

* update authors

* switch observable array format

* Small fixes here and there---nothing major.

* Update authors

* Address Chris` "Task" comments

* Further response to Chris' "Task" comments

* alphebetize and edit authors

* rename ResultBundle -> TaskResult

* Address Jim's comment

* Rename ObservablesTask -> EstimatorTask

* Update 0015-estimator-interface.md

Co-authored-by: Takashi Imamichi <31178928+t-imamichi@users.noreply.github.com>

* Update 0015-estimator-interface.md

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>

* Update 0015-estimator-interface.md

Co-authored-by: Takashi Imamichi <31178928+t-imamichi@users.noreply.github.com>

* Update 0015-estimator-interface.md

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>

* Update 0015-estimator-interface.md

Co-authored-by: Blake Johnson <blakejohnson04@gmail.com>

* fix typos in broadcasting examples

* Address Blake's comment about exponential memory overhead for BaseOperator

* Added Migration Examples section

This is following a request to see some simpler examples of what the new
interface would look like.

* Update 0015-estimator-interface.md

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* Update 0015-estimator-interface.md

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* Fix shapes

* Update 0015-estimator-interface.md

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>

* Removed 'Inner' from figure

* Update 0015-estimator-interface.md

Co-authored-by: Jim Garrison <jim@garrison.cc>

* Update 0015-estimator-interface.md

Co-authored-by: John Lapeyre <jlapeyre@users.noreply.github.com>

* Update 0015-estimator-interface.md

Co-authored-by: Toshinari Itoko <15028342+itoko@users.noreply.github.com>

* Update 0015-estimator-interface.md

Co-authored-by: Toshinari Itoko <15028342+itoko@users.noreply.github.com>

* Update 0015-estimator-interface.md

Co-authored-by: Toshinari Itoko <15028342+itoko@users.noreply.github.com>

* change migration path to versioning

* clarify output type

---------

Co-authored-by: Samantha Barron <sam@sambarron.me>
Co-authored-by: Takashi Imamichi <31178928+t-imamichi@users.noreply.github.com>
Co-authored-by: Luciano Bello <bel@zurich.ibm.com>
Co-authored-by: Blake Johnson <blakejohnson04@gmail.com>
Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>
Co-authored-by: Jim Garrison <jim@garrison.cc>
Co-authored-by: John Lapeyre <jlapeyre@users.noreply.github.com>
Co-authored-by: Toshinari Itoko <15028342+itoko@users.noreply.github.com>