Local DM Service: prepareData function linking operation

One of the compulsory services at the SRCNet v0.1 nodes is the Local Data Management Service, which will be in charge of prepare the datafile/s that is/are going to be analysed in the storage area attached to the local computing environment
This service will get as input:

• the ID (or IDs) of a specific datafile/s stored in the Rucio Data Lake
• the user token

with this input, the service will link the corresponding datafile/s into the scratch area attached to the computing environment that will be used to analyze it/them.

In particular the local DM will:

• find out the corresponding local user (from the user token)
• create a symbolic link in the user area attached to the computing environment. 

We can use CANFAR to do this implementation in this way:

• Assuming CANFAR PosixMapper maps the "network users" (whose userID can be get from the user token) to the local user ( same network user will always be mapped to the same local user), we can get the local path (at operating system level) of the user storage area.

• Assuming CANFAR mounts the Local RSE in read-only mode (this has been recently demoed by M. Parra), we can create a symbolic link (at operating system level) from the data file in the Local RSE volume to the local user storage area

This service will implement calls in line with the to be agreed interface defined at:

https://jira.skatelescope.org/browse/SP-4678