Introduce NumericTensor class

[asfimport]

#2759

This commit defines the new NumericTensor class as a subclass of Tensor class. NumericTensor extends Tensor class by adding a member function to access element values in a tensor.

I want to use this new feature for writing tests of SparseTensor in #2546.

Reporter: Kenta Murata / @mrkn
Assignee: Kenta Murata / @mrkn

Externally tracked issue: #2759

PRs and other links:

• GitHub Pull Request #2759

_{Note: This issue was originally created as ARROW-3515. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 2759
#2759