Use hour or day deltas to calculate overdue durations

Use hour or day deltas to calculate overdue durations

If durations are calculated by subtraction they will use units
larger than those we care about and these are not convertable
to the smaller units we are attempting to enumerate
Use the appropriate delta methods to calculate theee fines

Adds a separate hours_between method to calendar
This should strictly be checking opening hours (of which
closed days are a special case) of the relevant branch
These need adding to branches

http://bugs.koha-community.org/show_bug.cgi?id=7852
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Ian Walls <koha.sekjal@gmail.com>
QA comment: renamed "days_minus_grace" variable to "units_minus_grace"
Also added POD to _get_chargeable_units

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>