Use WebWorkers to improve loading performance

[NicolasRannou]

• Use web workers to see if loading performance can be improved.
• Get rid of Progress Bar in loader.

Findings:

• One worker per DICOM file is not a good solution since it takes about 10ms to instance a worker, it ends up being significantly slower.

Next step:

• Use a pool of workers (4?) to load the data. Those workers will only be initialized 1 time (initialization costs about 40ms) however that should then allow us to divide parsing time in 4.