Defines a function that allow to get/assign a global variable whose value is
ensured to be reset after each access.
Usage
oneoffVariable(default = NULL)
Value
a function with one argument (value) that provides get/set access
to a global variable.
If called with a value, it assigns this value to the global variable.
If called with no argument, it returns the current value of the global variable and
reset it to its default value -- as defined at its creation.
Arguments
default
default value to which the global variable is reset after each
access. Default is NULL.