Make streams configurable at dataset level

[NParsonsMO]

Raw data (pp files) appear in different streams in some runs.

The user should be able to override our defaults for a particular model run.

They will have to know where each mip table and variable combination (e.g. Amon/tas or day/pr) ended up (although can look at our default file).

They could then input a path to their own version of the file.

[NParsonsMO]

To test this functionality, the MJO recipe can be used.

1. It needs to use a branch of ESMValTool that has the MJO recipe.
   This branch is "3994-add-existing-mjo-recipes"
   Instructions for doing this are here: https://metoffice.github.io/CMEW/user_guide/esmvaltool_branches.html
   (You can do it in the Rose GUI).

2. "mjo" needs adding to the list of recipes in the flow.cylc.
   i.e. change recipe = radiation_budget to recipe = radiation_budget, mjo

3. Custom stream info needs adding for the runs u-az513 and u-bv526.
   The stream config is below (it's a change to day/pr and day/ua).
   Instructions for doing this are contained within this PR.
   It might be that we actually want those runs to be the default, and u-cw673 to be the exception?

The task compare_mjo will fail, but that's OK. We haven't set up a test to compare that output.

Stream config:

ap6:
  - day/pr
  - day/ua
apm:
  - Amon/hfls
  - Amon/hfss
  - Amon/rlds
  - Amon/rlut
  - Amon/rlutcs
  - Amon/rsds
  - Amon/rsdt
  - Amon/rsut
  - Amon/rsutcs
  - Amon/tas
  - Emon/rls
  - Emon/rss
inm:
  - SImon/siconc