Specify directory stucture per path instead of per project in config-user.yml

[bouweandela]

At the moment all drs need to be the same per project, while it would make much more sense to be able to specify the drs per rootpath.

To make this more clear, now we have this:

rootpath:
  # Rootpath to CMIP5 data
  CMIP5:
    - /badc/cmip5/data/cmip5/output1
    - ~/some_other_path

# Directory structure for input data: [default]/BADC/DKRZ/ETHZ/etc
# See config-developer.yml for definitions.
drs:
  CMIP5: BADC

while I think this would be much more useful:

# Directory structure for input data in the format
# input_data:
#   project_a:
#     /path/to/dir_1: drs_1
#     /path/to/dir_2: drs_2
#   project_b:
#     /path/to/dir_3: drs_2
# See config-developer.yml for valid drs definitions per project.
input_data:
  CMIP5:
    /badc/cmip5/data/cmip5/output1: BADC
    ~/some_other_path: default

[valeriupredoi]

slightly bit counter-intuitive since currently all the keys in config-user are system keys and their values are user-defined - @mattiarighi

[bouweandela]

Related to #181

[bouweandela]

I'll try to fix this for v2.8.

[valeriupredoi]

this is #1894 as it's linked by @bouweandela above

[bouweandela]

Bumping to the next milestone as there is no consensus on the implementation, see discussion in the linked pull request.