Plotting SP needs to be optimized.

[mrecachinas]

Describe the bug
Plotting scalar packed files on larger (~15 MB) files (i.e., overlay_href) is currently inefficient. Changing SP to SB on the server-side, while space-inefficient, speeds up plotting. Also, plotting SP blocks the entire page, so if you have multiple plots and the SP overlay_href triggers first, none of the other plots will render until SP is done.

[mrecachinas]

Updates from profiling and discussing with @maihde:

• A lot of time is spent in bluefile.js:BitArray and vlogscale
• It's odd to plot a SP as log scale (what does that mean for 0s and 1s?)
  • Workaround for this is to try setting cmode: 3
• Could optimize by optimizing bluefile.js:BitArray.subarray or reducing number of times it needs to be called
  • Could rewrite BitArray in WASM.
• Browser is locking up because overlay_bluefile takes 6 seconds to execute.
• Another option to alleviate locking is to make functions like create_image work in chunks and release the control thread back periodically.
• Long term solution is to use WebWorkers for all non-rendering funcs.

[mrecachinas]

Effort in https://github.com/LGSInnovations/sigfile to integrate BitArray WASM.