Mlj.jl: Add Tables 1.0

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This should be a mostly pain-free addition with the only expected sticking point being Tables.matrix which may have changed its behaviour slightly.

Plan:

• [x] get prettytables to use 1.0
• [x] Update MLJScientificTypes's compat bound to Tables 1.0
• [x] Check MLJModelInterface goes well with previous update (merged https://github.com/alan-turing-institute/MLJModelInterface.jl/pull/22)

Note: for subsequent points, first box is for when the patch release is done

• [ ][x] Update MLJBase's compat bound to Tables 1.0 (PR https://github.com/alan-turing-institute/MLJBase.jl/pull/204)
• [ ][x] update MLJModels' compat bounds and check doesn't complain (PR https://github.com/alan-turing-institute/MLJModels.jl/pull/193)
• [ ][x] idem with MLJTuning (PR https://github.com/alan-turing-institute/MLJTuning.jl/pull/16)
• [ ][x] Update MLJ's compat bound to Tables 1.0 (PR https://github.com/alan-turing-institute/MLJ.jl/pull/448)

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There's something weird with Tables.matrix so this is momentarily paused again. https://github.com/JuliaData/Tables.jl/issues/155

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I'm removing the low-priority label; for a couple of reasons we want this to be done fairly soon:

1. MLJScientificTypes 0.2 depends upon it and implements useful fixes, it would be annoying to backport those to 0.1 (e.g. the ._table field in schema, the Meta.parse etc)
2. DataFrames 0.20 depends upon it and we've typically had issues in the past when not moving fast enough with DataFrames and users complaining of compat issues

Provided the issue quoted above is solved soon, this should be a reasonably light undertaking.