Roviz: limit the number of visualized results

[seba-1511]

Until a more efficient visualization template is written, it is difficult to justify loading more than 1000-2000 experiments.

So introduce the environment variable ROVIZ_NRESULTS which is equal to 1000 by default, and indicates the max number of results to be included in the visualization.

[seba-1511]

In hindsight, this is a bad idea. Instead the web app should be made efficient enough to handle large collections of results.