Implement efficient data utils methods for Tables.MatrixTable

[OkonSamuel]

There is a need to implement efficient selectrows and selectcols methods for Tables.MatrixTable table type. The current data utils methods only have efficient fallbacks for DataFrame table type, but we could easily add efficient methods for Tables.MatrixTable type after this issue get resolved.
This would avoid materializing wide matrix tables as a NamedTuple which the Julia compiler has issues handling.

cc. @ablaom

[ablaom]

Agreed.

Waiting for:

• resolution of materializer for matrix tables JuliaData/Tables.jl#281