Required fixes/updates to core CORDEX notebooks

When setting up the new AFR-22 notebooks, a number of issues/potential improvements to the core EAS-22 notebooks have been identified

## errors

- [x] Inconsistent location for location of data (historical in WS3 but not WS2) #138
- [x] 4.2c solution is for pr not tm and also only historical not future as well #139
- [x] Wet days - using threshold of 0 not 1 -replace custom aggregator with count > 1 #140

## Improvements

- [ ] S3 and rsync - need to have clarity on which using in standard and a subsidary one with other options #145
- [x] Future dir should be explicitly rcp85
- [x] Mean over season should be over time (this should remove redundant time dimension)
- [x] Inconsistent on use of REMO2015 in filenames
- [x] Needs to be made VERY clear when we expect something will fail (worksheet 1 - merge problem)
- [ ] follow approach of scitools/iris course for examples use e..g %load solutions/iris_exercise_1.2a #141

## Simplify
- [ ] Remove slicing section from 2.1b
- [ ] Reduce the section on shell commands - wc -l not essential for example. Consider moving out of a notebook as well
- [ ] Add plot of country X with coastlines and zoom to notebook 1
- [x] Remove f strings and remaining .format (just use concatenation) - no need to learn this
- [ ] Remove tuples - just an extra thing to learn, all can be done with lists

## For adaptability

- [x] set domain name as variable at start
- [ ] Always call data directory just data (population of this dir is part of the customisation per course depending on logistics)
- [ ] Variable for city name and location