Performance variability boxplots

[jarusified]

This PR migrates the performance variability boxplots from CallFlow into Hatchet's jupyter notebook interface through the roundtrip. Merging of this PR is blocked by [todo]

The following have been added to the hatchet's codebase.

1. VIS code to render boxplots. Refer hatchet/external/roundtrip/boxplot.js.
2. D3 utility code to support rendering operations through an SVG element. These utilities generalize several rendering functions to the jupyter element. Refer hatchet/external/roundtrip/lib/d3_utils.js.
3. A sample notebook showcasing the functionality. Refer docs/examples/tutorial/performance_boxplot.ipynb. For a 2-min demo, see video.
4. Improvements to roundtrip interface to generalize the %loadVisualization and %fetchData APIs for other any visualization. Refer hatchet/external/roundtrip/roundtrip.py.

User interface

# Load roundtrip
%load_ext roundtrip  

# Boxplot data computation 
bp = BoxPlot(tgt_gf=gf, bkg_gf=None, callsites=callsites, metrics=["time"])

# Dump the data as JSON.
boxplot = bp.unpack() 

# Load visualization for boxplots. 
%loadVisualization roundtrip_path "boxplot" boxplot_data

# Fetch data from Visualization.
%fetchData "boxplot" variance_df

# The data in the vis is dumped as a csv text. (columns separated by `,` and rows separated by `;`. 
# This data can be converted to a Pandas dataframe using the below code. 
columns = variance_df.split(';')[0].split(',')
data = [x.split(',') for x in variance_df.split(';')[1:]]
df = pd.DataFrame(data, columns=columns).set_index('name')

Schema for boxplot_data:

{
  "callsite1" : {
      "tgt" : {
          "metric1": {
              "min": number,
              "max": number,
              "mean": number,
              "imb": number,
              "kurt": number,
              "skew": number,
              "q": [q0, q1, q2, q3, q4],
              "outliers: {
                  "values": array,
                  "keys": array
              }
          },
          "metric2": {
              ...
          }
      },
      "bkg": {
          // Refer "tgt" key.
      }
  }, 
  { 
     "callsite2": {
      ...
      }
  }
}