add an list of ALL metric grades

Update other classes to extend the new one.
add a script to update the total marks on page metric grade records
Cache the number of notices and errors for page metric grades
Store the mark totals on the page_metric_grades record

This should make it easier to access (less complicated and faster sql queries)
add some tests
Add a script to update the mark totals for metric grades
add the mark total fields to the MetricGrade record class
Simplify the hotspots query
Merge pull request #80 from mfairchild365/metric-grades-update

Order hot spots by number of errors, not number of total marks