[GSoC] Issue with compute_timeseries method in dataframe reference implementations

[Polaris000]

compute_timeseries method in the commit-related metric classes can be improved. The main issue is not exactly with the current implementation of compute_timeseries, rather, It is because the self.since and self.until parameters have to be set to particular values. This leads to them being assigned values in the __init__ method of the classes.

[Polaris000]

The current implemention of compute_timeseries of Code_Changes metric is like this:

• create a dataframe by grouping data based on year, followed by month of creation and then aggregating them with count.
• create a dataframe with rows representing every possible interval of time between the since and until parameters. For example, if the period is "month", then the second dataframe will have each year, followed by each month for that year between the date_range dates as rows.
• merge the two dataframes above

It is clear that creating the second dataframe will require a definite since and until date. The advantage of this method is that it allows for easier plotting of graphs, say, the number of commits per month. Only grouping the first dataframe would mean that months without commits will not be included at all, making a plot based on this dataframe skewed.

[Polaris000]

Closing this since this was addressed in #176