Add orbital numbers file parser and Events trimmer

Added

• New OrbitEventsFile parser that can used with read_events() (#39 (closed)):

>>> orbits = read_events('juice.orb')

no.	event utc peri	event sclk peri	...	semi axis
1	2031 JUL 21 20:46:34	1/0288391662.57647	...	-8734123.78
2	2032 FEB 14 14:19:45	1/0306339653.54137	...	4313415.24

The orbits can be queried by No. values:

>>> orbits[1]  # returns a Event object

no.	1
event utc peri	2031 JUL 21 20:46:34
event sclk peri	1/0288391662.57647
...	...
semi axis	-8734123.78

Support for tuple and slice is also implemented:

>>> orbits[2, 4]

	no.	event utc peri	event sclk peri	...	semi axis
0	2	2032 FEB 14 14:19:45	1/0306339653.54137	...	4313415.24
1	4	2032 MAY 10 14:44:53	1/0313771562.20434	...	2298334.65

>>> orbits[1:2]  # Note: the last element is always included

	no.	event utc peri	event sclk peri	...	semi axis
0	1	2031 JUL 21 20:46:34	1/0288391662.57647	...	-8734123.78
1	2	2032 FEB 14 14:19:45	1/0306339653.54137	...	4313415.24

Therefore it is fairly easy to compute a trajectory for a given orbit number:

>>> traj = tour[orbit[1:2]]  # or tour[orbit[1]:orbit[2]] -> start='2031-07-21T20:46:34' and stop='2032-02-14 14:19:45'

• Event, EventWindow, EventsList and EventsDict objects can be trimmed with time boundary conditions (#58 (closed)):

>>> event = EventWindow('FOO', t_start='2035-04-01', t_end='2035-07-01')

>>> event.trim(before='2035-05-01')
<EventWindow> FOO (2035-05-01 -> 2035-07-01):
 - t_start: 2035-05-01
 - t_end: 2035-07-01

>>> event.trim(after='2035-06-01')
<EventWindow> FOO (2035-04-01 -> 2035-06-01):
 - t_start: 2035-04-01
 - t_end: 2035-06-01

>>> event.trim(before='2035-05-01', after='2035-06-01')
<EventWindow> FOO (2035-05-01 -> 2035-06-01):
 - t_start: 2035-05-01
 - t_end: 2035-06-01

# trim by Event/EventWindow object
>>> event_bar = EventWindow('BAR', t_start='2035-05-01', t_end='2035-06-01')

>>> event.trim(by_event=event_bar)
<EventWindow> FOO (2035-05-01 -> 2035-06-01):
 - t_start: 2035-05-01
 - t_end: 2035-06-01

# Outside the time window -> return `None`
>>> event.trim(before='2035-08-01')
None

>>> event.trim(after='2035-03-01')
None

In all cases, you get a copy of the event(s) with the same the primary key and properties but with a trimmed start and stop time. The original events are not edited.

Fixes

• datetime() parser now support NaT/N/A values.

---

Closes #39 (closed) and #58 (closed)