Clean up tables interface implementation

[quinnj]

Reasoning for changes:

• We don't need to define rowaccess/rows because if we define columnaccess/columns then we automatically get an efficient fallback rows definition
• We don't need to define columnnames/getcolumn/schema on TimeArray, only on the type that Tables.columns returns, so in this case, just TableIter
• In the constructor, we need to call Tables.columns first and then call Tables.schema(cols); calling Tables.schema directly on the table input isn't valid, as it wouldn't work in, for example, the case where someone passed in a ::TimeArray.

[iblislin]

The reason that we defined schema is for the time column. Since the fallback function return only the data column. for example, [:a, :b, :c], but we defined the custom schema to return [:timestamp, :a, :b, :c]

[quinnj]

The reason that we defined schema is for the time column. Since the fallback function return only the data column. for example, [:a, :b, :c], but we defined the custom schema to return [:timestamp, :a, :b, :c]

Yep, that's perfectly fine/valid.