[Complete] Simple replication

[omad]

Add a simple replication tool.

There's masses of room for improvement, but I found this useful for downloading data from the AGDC to my local PC before outages at the NCI.

[mpaget]

@omad: could you add some notes to documentation/Components and Interfaces.md > Managed Replica reflecting what this commit does and does not do relative to the intentions of the general solution, please?

[coveralls]

Coverage remained the same at 81.239% when pulling 7ca43ae on simple_replication into 58a8fcf on develop.

[coveralls]

Coverage remained the same at 81.239% when pulling 02fcdbc on simple_replication into 58a8fcf on develop.

[andrewdhicks]

Very clear and straight-forward code. Good documentation.

It is specific to file-based storage systems, so we may have to modify how it works when we abstract the storage mechanism to a driver-based model.

@mpaget I'll add a link to the simple replication readthedocs page to the components page, as a reference point for the implementer of the managed replica components.