MAGMA-172: Add method to obtain the duration between two dates or two datetime (maybe add support to minus method)

[ymarcon]

Jira issue originally created by user @plaflamme:

Overload the minus method:
$('DATE').minus($('ANOTHER_DATE'))
or create a new specific method:
$('DATE').duration($('ANOTHER_DATE'))

[ymarcon]

Comment created by @plaflamme:

The result should have the proper unit (seconds for example) this way, one could convert them to other units:

// Compute the duration (in seconds) and convert to hours
$('DATE').duration($('ANOTHER_DATE')).toUnit('h')