notebook clearing

[mnfienen]

in PR #102 I added a script notebooks/clear_all_output.py that will clear all notebook output unless 0) it's in the list inside the script of excluded nbs or 1) it's in the solutions folder. So probs good if we run the script prior to making PRs. So please add notebooks you want output preserved for to the list in the script.

[mnfienen]

all sorted