Publish lessons - additions from UCL chemistry

[mParkesUCL]

Some edits and changes made at UCL focussing on the variables section working with @m-rivera and our summer student Tara

Main alterations -

• We have used HTML colours within the mathematical_operators notebook to create admonitions and indicate exercises and different regions of the tutorial. These are a pure HTML implementation so should also display correctly if the notebook is outside of the static pages. The style was based on an existing one in the existing lesson on loops. If you are happy we will apply to our other notebooks
• Have split up the comparisons and conditions lessons into two. Comparisons and booleans stay in the section on variables and operators. Conditions make sense in the section on loops and functions. Which should be renamed as structing code as organising perhaps suggest how to organise the files for the code
• Various small typos/errors that jupyterbook didn't like