Final changes to MPI material

Change log:
* simplify heat equation exercises by providing partial solution from the beginning
* rearrange low-priority material to the end of the lesson

Create dedicated heat equation notebook for MPI optimisations. Link to
heat equation notebook from main notebook.

Change log:
* add heat equation code base
* add notebook to edit and run heat equation code from
* label heat equation plots
* minor changes

Additional exercises. Some polishing

Change log:
* remove exercises pdf
* add file write and invocation for exercises
* fix missing enforcement of two processes run of Send_Recv.py
* add allocation times for exercises
* change second pairwise exchange exercise to combined sendrecv instead
  of non-blocking
* add empty-ish code blocks for modification exercises for better
  clarity
* add heat equation optimisation exercises

Create subtree to sync with public repo and configure update script to
perform sync.

Change log:
* remove Online directory
* update remote repo update script to reflect new format
* replace old remote target for update script

git-subtree-dir: march-2021
git-subtree-split: 50a20c02b790bd122e7963312b53ddce112b96a3

Add on-the-fly exercises to course material and some other minor changes.

Change log:
* exercise: blocking pairwise exchange
* exercise: point-to-point partial sum
* exercise: non-blocking pairwise exchange
* exercise: collective communication partial sum
* combine MPI notebooks
* provide exercises in notebook and pdf formats
* Fix markdown table formatting in exercises notebook
* polishing
* flesh out skeleton code

James cython

See merge request training/hpc-python!4

Chris W merge

See merge request training/hpc-python!3