default engine changes for #513

[topepo]

Each model now has a default engine that is used when the model is defined. The default for each model is listed in the help documents.

This also adds functionality to change to declare an engine in the model specification function.

set_engine() is still required if engine-specific arguments need to be added

[juliasilge]

This is a really nice change; I am super happy about it.

I just made a few edits to how we describe mode and engine so they are more consistent and more in line with how we talk about them elsewhere, like here and here. For example, I don't think we should use the word "type" for either of those since we use it for the algorithm choice.

[juliasilge]

I can't imagine there will be any problems but have any of us run extratests with this PR built?

[topepo]

I can't imagine there will be any problems but have any of us run extratests with this PR built?

[ FAIL 0 | WARN 0 | SKIP 6 | PASS 252 ]

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