Use of "heatmap" in code

[egpbos]

Following #201, we should also replace use of the term heatmap in code, where relevant. After #210, some 50 uses of "heatmap" remain, which can be found with the following grep:

grep -r heatmap . --include='*.py' --include='*.ipynb' --exclude='./.tox/*' --exclude='*checkpoints*' --exclude='./build/*'

The goal is to have self-documenting code which is in sync with the actual documentation. Note that although in dianna/visualization/image.py we indeed are generating heatmaps as output, the input parameter should still be called relevance score as well (since we are not a plotting package which does generic heatmap plotting, that's what matplotlib is for). It does make sense to mention in the docstring that the output is a heatmap, of course.