Add comparison operations on the xios_duration and xios_date types:
xios_duration: ==, /=
xios_date: ==, /=, <, <=, >, >=
Add a new function "xios_date_convert_to_seconds" to convert a date into the number of seconds since the time origin of the calendar
Define some constant durations "xios_second", "xios_minute", "xios_hour", "xios_day", "xios_month", "xios_year" et "xios_timestep" to ease the definition of new durations (for example, 10h is just 10 * xios_hour)
Add a new function "xios_set_calendar" so that one can manually create the calendar attached to the current context and thus use the calendar operations before calling "xios_close_context_definition". This function can accept optional parameters so that the calendar attributes (calendar_type, start_date, time_origin and timestep) can be easily overwritten. Note that you cannot define a new calendar after one was already created (either because "xios_set_calendar" or "xios_close_context_definition" was used)
Readd the function "xios_set_timestep" as a simplified alias of "xios_set_context_attr(context, timestep)" for the current context