Add support for external data

[agarny]

We want to be able to upload some external data (from either a local file or a remote file) and use that data to create traces. The data file is expected to be a CSV file. For instance:

voi,data_1,data_2,data_3
0,1,1,1
0.001,1.00013000900608,1.02598903969544,0.998348641336412
0.002,1.00051672500024,1.0519607453206,0.996727835285807
0.003,1.00115801085363,1.0779218541107,0.99513783069357
...
49.998,1.41872900414933,2.56233550286678,10.8491477253401
49.999,1.43021691799542,2.58421103556028,10.8239158391742
50,1.44180924652132,2.60629879462131,10.7988123362029

• We need to think about the fact that voi may not match the VOI values from a given simulation, so we should be able to transform voi if needed (e.g., real_voi = 0.001*voi).
• We should be able to plot any external data against any other external data OR any model parameter.