Exact stratified sample

[Jday7879]

Merge request template: please remove the appropriate parts of this template.

Pre-merge request checklist (to be completed by the one making the request):

• [x] I have performed a full review of this code myself.
  • For Python code in PySpark specific sections, all code should have been run in Jupyter notebooks.
  • For code in sections of the book containing both Python and R code, the page of the book should be constructed as described in the contributing guide and converted to a markdown file.
• [x] I have formatted the outputs of code blocks correctly (to match other outputs in the book and in line with the style guide [coming soon])
• [x] I have built the book as outlined in the contributing guide and confirmed that any additional/modified content is displaying as expected.

Details of this request (such as):

• Extended the sampling page to include a method for returning an exact sample for stratified sampling
• This contains both pyspark and sparklyr code
• Page has been converted into the markdown file

Things to note about this request (such as):

• One small change has been made to the reading of the csv for sparkly r, otherwise all older sections are unchanged.
• Because it uses a random uniform dist for sampling, outs will have changed between the old and new versions of the page.

closes #102

[Jday7879]

Book builds correctly with new section using tibble to simplify the creation of the 'sample_size' column

[NathanKelly-ONS]

Changes look good, I've built the book and verified that it's displaying correctly so will go ahead and merge.