Support for DateTime time interval arithmetic

[alopezrivera]

Rationale

Tudat DateTime objects are useful for date input in programs/applications using tudat or tudatpy.

For many applications, such as for example porkchop mission design plots, it is commonplace to use as inputs

• A departure time window
• Accompanied by a minimum and maximum flight time

It would be very convenient to facilitate this type of input with tudat DateTime objects.

Request

A syntax for time intervals in Tudat which can be added or subtracted from tudat DateTime objects.

Sketch:

from tudatpy.kernel.astro.time_conversion import DateTime, TimeSpan

earliest_departure_time = DateTime(2005,  4,  30)
latest_departure_time   = DateTime(2005, 10,   7)

earliest_arrival_time = earliest_departure_time + TimeSpan(2, 'months')
latest_arrival_time = earliest_departure_time + TimeSpan(6, 'months')

[DominicDirkx]

This functionality has been added, although in a slightly different form. Since months and years are never the same length, there are now two functions, one to add a number of seconds, and one to add a number of days (86400 seconds):

from tudatpy.kernel.astro.time_conversion import DateTime, TimeSpan

earliest_departure_time = DateTime(2005,  4,  30)
latest_departure_time   = DateTime(2005, 10,   7)

earliest_arrival_time = add_seconds_to_datetime( earliest_departure_time, 2.0 * 30.0 * 86400 )
latest_arrival_time = add_seconds_to_datetime( earliest_departure_time, 6.0 * 30.0 * 86400 )

earliest_arrival_time_alt = add_days_to_datetime( earliest_departure_time, 2.0 * 86400 )
latest_arrival_time_alt = add_days_to_datetime( earliest_departure_time, 6.0 * 86400 )