[MS2/metrics/github_issues] Add project process functions & Metrics for github_issues

[aswanipranjal]

This PR:

• updates the calculate_bmi function to handle dataframe timeseries data and calculate the ratio of closed issues/pr to opened issues/pr

• Adds project_process function for the github_issues data source

• Updates DaysToCloseMedian timeseries method to return the values as float values with precision upto 2 digits after the decimal

• Adds DaysToCloseAverage class which calculates the average values for time to close days values of closed issues. It has a timeseries method same as that of DaysToCloseMedian class.

• Adds tests and data for the BMI, DaysToCloseMedian and DaysToCloseAverage classes.

[aswanipranjal]

@valeriocos please review?

[coveralls]

Pull Request Test Coverage Report for Build 258

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 16 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.4%) to 61.894%

---

Files with Coverage Reduction	New Missed Lines	%
/home/travis/build/chaoss/grimoirelab-manuscripts/manuscripts2/elasticsearch.py	8	85.9%
/home/travis/build/chaoss/grimoirelab-manuscripts/manuscripts2/metrics/github_issues.py	8	90.63%
<!--	Total:	16		-->

Totals
Change from base Build 257:	1.4%
Covered Lines:	1111
Relevant Lines:	1795

---

💛 - Coveralls