Update EDITS-MCE scenario labels and description

[r-aneeque]

• Change EDITS-MCE scenario labels to
  [SSP2] BAU
  Step 1: + Shared mobility supply
  Step 2: + price incentives
  Step 3: + car restraint
  Step 4: + Maas
  I prefer descriptive names; however, that misses the point that each step is additional to the one before. So, if the full names are too long, we can also go with the Step1 etc.
  I put SSP2 in the brackets, since we will use the techno-economic assumptions and other variables not provided by PASTA from SSP2 scenario. However, the service demands for BAU from PASTA will be different from our SSP2 service demands, so not our standard SSP2 setup either.

• Add a description and mechanism of implementation (levels as well as the variables) so that they can be altered/used in MixT in the future.