Generalize Transformation API

[pulsipher]

This replaces the optimizer model abstraction which was limited to JuMP Models with a new abstraction based on what is called transformation backends. This introduces AbstractTransformationBackend to enable the use of arbitrary backends for solving InfiniteModels. For backends that use JuMP, this also adds JuMPBackend <: AbstractTransformationBackend which provides a similar API to the old optimizer model interface except it is more polished and doesn't rely on hacky "optimizer model keys".

TranscriptionOpt now only exports TranscriptionBackend which is based on JuMPBackend. All the former TranscriptionOpt specific query functions have been dropped from main API in favor of the general purpose ones (e.g., transformation_variable).

This also adds a few missing JuMP methods which include:

• set_attribute
• get_attribute
• unsafe_backend
• compute_conflict!
• copy_conflict
• set_string_names_on_creation
• print_active_bridges
• print_bridge_graph

It also cleans up the documentation and bit and uses the latest Documenter version.

Partly addresses #105 and #327. What remains for #105, is to decouple the transcription specific approaches from InfiniteOpt. This will require the following:

• Refactoring the measure abstraction to be agnostic of the backend
• Moving the derivative evaluation methods to TranscriptionOpt
• Moving the measure expansion methods to TranscriptionOpt

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.83766% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.77%. Comparing base (6e1102d) to head (85c3ab5).

Files	Patch %	Lines
src/TranscriptionOpt/transcribe.jl	99.21%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #349 +/- ## ========================================== + Coverage 99.74% 99.77% +0.03% ========================================== Files 36 35 -1 Lines 6703 6712 +9 ========================================== + Hits 6686 6697 +11 + Misses 17 15 -2 ```

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[pulsipher]

For now, I am going to turn off the checkdocs since it appears to have a bug: https://github.com/JuliaDocs/Documenter.jl/issues/2545