Change unpack to avoid colname -> true mystery

[ablaom]

A rather sophisticated user found the current behaviour of unpack difficult to understand, despite it's detailed (but possibly deficient) docstring.

I also find it annoying to include _ -> true to pick up all remaining columns for the last return value when this is what I want 90% of the time anyway.

Finally, the current design is inconsistent with partition: there you specify n - 1 fractions to get n return values, whereas unpack is requiring n filters for n return values.

I propose that unpack(data, filters...; kwargs...) return what, under the current implementation, is returned by unpack(data, filters..., _ -> true; kwargs...). That is, the last return value is always "everything left over".

So, for example, if I have a table - with columns :a, :b, :c and :d - and I want to unpack column :b as y and the rest as X then the new syntax would be

y, X = unpack(data, ==(:b))

If instead X should exclude :d (ie, include only :a and :c) then I would do

y, X, _ = unpack(data, ==(:b), !=(:d))