get.grades: Calculate the grades according to the Records
In wush978/DSRTeacher: Export Tracking Data of Students from DataScienceAndR

1 2	get.grades(result, grades, thresholds, decay.gap = 86400 * 7, decay.ratio = 0.2)

`grades`	A named numeric vector. The name is the course name. The value is the grades associated with the course.
`decay.gap`	The duration of discounting the score if the student is late. Please see the details. Specifying `decay.gap = 0` will make the score directly be 0 if the student is late.
`decay.ratio`	The amount of discounting of the score if the student is late.
`records`	The data from `get.progress.table`
`threshold`	A POSIXct vector. This is the deadline of the corresponding course / grades.

If the student was late but he/she accomplished the course in decay.gap seconds after the threshold timestamp, then his/her score was multiplied by 1 - decay.ratio. The discount will become heavier and heavier until the ratio become 0 or less.

A data.frame whose column is:

student The classroom account of the student.
grades The total scores of the student.
comment The detailed score of the student.

## Not run: 
# login first
records <- get.records()
result <- get.progress.table(records)
grades <- structure(c(0, 6, 0, 4, 0, 0, 6, 2, 2, 2, 2, 0, 4, 4, 4, 0, 6,
 6, 3, 3, 3, 3), .Names = c("2017-NTUCSX:2nd-week", "2017-NTUCSX:2nd-week-hw",
 "2017-NTUCSX:3rd-week", "2017-NTUCSX:3rd-week-hw", "DataScienceAndR:00-Hello-DataScienceAndR",
 "DataScienceAndR:01-RBasic-01-Introduction", "DataScienceAndR:01-RBasic-02-Data-Structure-Vectors",
 "DataScienceAndR:01-RBasic-03-Data-Structure-Object", "DataScienceAndR:01-RBasic-04-Factors",
 "DataScienceAndR:01-RBasic-05-Arrays-Matrices", "DataScienceAndR:01-RBasic-06-List-DataFrame",
 "DataScienceAndR:01-RBasic-07-Loading-Dataset", "DataScienceAndR:02-RDataEngineer-01-Parsing",
 "DataScienceAndR:02-RDataEngineer-02-XML", "DataScienceAndR:02-RDataEngineer-03-JSON",
 "DataScienceAndR:02-RDataEngineer-04-Database", "DataScienceAndR:02-RDataEngineer-05-Data-Manipulation",
 "DataScienceAndR:02-RDataEngineer-06-Join", "DataScienceAndR:03-RVisualization-01-One-Variable-Visualization",
 "DataScienceAndR:03-RVisualization-02-Multiple-Variables-Visualization",
 "DataScienceAndR:03-RVisualization-03-ggplot2", "DataScienceAndR:03-RVisualization-04-Javascript-And-Maps"
))
thresholds <- c("2017-03-16 08:00:00", "2017-03-16 08:00:00", "2017-03-16 08:00:00",
 "2017-03-23 08:00:00", "2017-03-16 08:00:00", "2017-03-16 08:00:00",
 "2017-03-16 08:00:00", "2017-03-23 08:00:00", "2017-03-23 08:00:00",
 "2017-03-23 08:00:00", "2017-03-23 08:00:00", "2017-03-16 08:00:00",
 "2017-03-30 08:00:00", "2017-03-30 08:00:00", "2017-03-30 08:00:00",
 "2017-03-16 08:00:00", "2017-04-06 08:00:00", "2017-04-06 08:00:00",
 "2017-04-13 08:00:00", "2017-04-13 08:00:00", "2017-04-13 08:00:00",
 "2017-04-13 08:00:00")
thresholds <- as.POSIXct(thresholds)

result.grades <- get.grades(result, grades, thresholds)

## End(Not run)