Refactor MetricsItem

[vijayiyer05]

The public access for the info and id properties seems like a code smell.

A couple of ideas:

1. Making this a "MetricsItem" class than inherits from Item. Currently there isn't a class that would inherit from both LinkedFilesItem and MetricsItem, so there isn't strictly a need for a mixin (i.e. multiple inheritance)
2. Using an access control list to specify which mixin(s) have access, if a good way can be identified to combine children & specified mixins.
3. Combining 1&2 to create a MixinItem abstract class of which LinkedFilesItem and MetricsItem would be subclasses; and then Item would give access just to MixinItem

Based on original post by @vijayiyer05 in #62 (comment)