utils/trace: Add squash_df() method

This method takes in a dataframe with a time delta column (as created by
addEventsDeltas), and slices it to fit within a given interval.

Instead of writing a wall of text, here's what it's doing:

df is:
Time len state
15    1   1
16    1   0
17    1   1
18    1   0
-------------

squash_df(df, 16.5, 17.5) =>

Time len state
16.5  .5   0
17    .5   1

squash_df(df, 16.2, 16.8) =>

Time len state
16.2  .6   0