Refactored saving bitmap files
Implement assets for MultiPlot
Add MultiPlot demo

For bitmapped 2d histograms
Fix demo TeX
Do not concern with implementation

MultiPlot should not need to know that we need to save bitmaps, and
possibly other types of assets. Just call save_assets.
We change CWD, no need for build flag

We need to change CWD because LaTeX cannot find the input files
otherwise. There is a -output-directory flag, but no easy
-input-directory flag.
Updated docstrings
Use less bins in the Histogram 2D demo

These plots took about a minute to make,
more because the pdf is built twice on the first demo run..
Which is too long, this halves (~225/400) that time, acceptable or still to slow?
PEP8 fix and change line flow
Merge pull request #1 from 153957/hist2d-assets

Merge refactoring of Histogram 2D assets and support for MultiPlot