Combining/Concatenating/Appending Axes

[Tokazama]

• Finalize design:
  • Combine continuous collections (i.e., ranges):
    • Solution: grow from the last position (e.g., append_axis!(x, y) and cat(x, y) will grow from the last position of x.)
  • Combine discrete (non-continuous) collections:
    • Stack unique elements and make non-unique elements unique similar to how DataFrames.jl does
    • If there is no idiomatic solution then maybe just error and allow custom axis types for special use cases.
• Document axis interface
• Document customization with axes
• Multidimensional (AbstractAxisIndices) examples

Biggest problem here is handling non-discrete values. I would much prefer a solution that doesn't default to an error and require users to create a custom axis type, but we have to ensure the solution doesn't hinder performance, make it difficult to understand for new users, or cause unexpected output.

[Tokazama]

v0.2.0 begins to address this implementation more fully, but it still needs to be crowd tested before I feel like it's ready to finalize the examples and documentation.