Add version warnings

[pavelzw]

No description provided.

[github-actions]

(benchmark 4439466212 / attempt 1)
Base results / Our results / Change

Model	Size	Dump Time	Load Time
RandomForestRegressor	77.2 MiB / 12.7 MiB / 6.08 x	0.07 s / 0.10 s / 1.42 x	0.08 s / 0.09 s / 1.07 x
GradientBoostingRegressor	2.2 MiB / 1.1 MiB / 2.12 x	0.03 s / 0.16 s / 5.31 x	0.03 s / 0.11 s / 3.34 x
LGBMRegressor gbdt	5.3 MiB / 1.9 MiB / 2.74 x	0.17 s / 0.39 s / 2.34 x	0.03 s / 0.30 s / 11.80 x
LGBMRegressor gbdt large	5.3 MiB / 1.9 MiB / 2.74 x	0.16 s / 0.39 s / 2.37 x	0.03 s / 0.30 s / 11.41 x
LGBMRegressor rf	1.6 MiB / 0.5 MiB / 3.30 x	0.05 s / 0.08 s / 1.83 x	0.01 s / 0.07 s / 12.68 x

[YYYasin19]

We could think about matching just the first two version numbers here, e.g. 2.3.1 and 2.3.2 would still work together. Otherwise, every minor patch and release causes all existing models to be invalid, although we didn't change anything on the compression side of things.

The schema would be
A.B.C where
C = some utils changed, nothing user-facing, nothing regarding the data, e.g. just speed-up
B = something with the compression changed
A = something with the user interface changed

I'm happy to merge like this right now, though, since we're in an early alpha and shouldn't have to worry about this too soon. :)

[YYYasin19]

nice 🎉