QST: Date Overlapping Ranges
I've been trying to learn how to use Pandas, but I'm thoroughly confused about where in the API to find methods that can aggregate data conditionally based on sign across date ranges. I have a data frame like so:

```
Date        Change 
2010-08-25    0.08
2010-08-26   -0.22
2010-08-27    0.04
2010-08-30   -0.08
2010-08-31   -0.11
...            ...
2020-08-18    0.96
2020-08-19   -1.79
2020-08-20    5.04
2020-08-21   -0.84
2020-08-24   -1.10
```
The Date column is an index of course. What I want to do is basically partition this data by year. Once partitioned by year, I want to group consecutive rows by the sign of the change column such that consecutive negatives and consecutive positives are grouped together. Once that is done, I want to get the overlap of date ranges with matching sign for all years. For example, if change is positive from 2010-08-25 to 2010-08-27 and from 2011-08-26 to 2011-08-29 the common overlap would be 08-26 to 08-27, obviously accounting for all years not just 2. At that point, once I have the common date ranges and their values I want to average all of the numbers in that range such that at the end I have ranges of dates for which change is consistently positive or negative and the average change for each range. How can I achieve this?
