Starting to refactor Storage and Operations

[jdunkerley]

Pull Request Description

Cleaning up some of the structures in Storage before working on UnaryOperations.

• Removed some legacy code: countMask, Index and DefaultIndex.
• Renamed mask to applyFilter on Column and Storage.
• Renamed Table.mask to Table.filter.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.