Add HTML representation to Event objects

In Jupyter, it is possible to implement _repr_html_ function to force the display of object in a custom HTML format.

For examples, for the event, the output could be represented as:

>>> phases

	event	#	t_start	t_stop
0	Approach and first ellipse	-	2031-01-19	2032-02-08
1	Energy reduction	-	2032-02-08	2032-06-25
2	Europa flybys	-	2032-06-25	2032-07-24
3	High-latitude	-	2032-07-24	2033-08-18
4	Low energy	-	2033-08-18	2034-12-19
5	All Jupiter phases	-	2031-01-19	2034-12-19
6	GEOa	-	2034-12-19	2035-01-17
7	GCO5000	-	2035-01-17	2035-04-16
8	GEOb	-	2035-04-16	2035-05-13
9	TGCO	-	2035-05-13	2035-05-21
10	GCO500	-	2035-05-21	2035-09-29
11	All Ganymede phases	-	2034-12-19	2035-09-29
12	All mission phases	-	2031-01-19	2035-09-29

instead of:

<EsaMissionPhases> Mission_Phases-Crema_5.0.csv (2031-01-19 -> 2035-09-29 | 13 keys):
 - Approach and first ellipse (2031-01-19 -> 2032-02-08)
 - Energy reduction (2032-02-08 -> 2032-06-25)
 - Europa flybys (2032-06-25 -> 2032-07-24)
 - High-latitude (2032-07-24 -> 2033-08-18)
 - Low energy (2033-08-18 -> 2034-12-19)
 - All Jupiter phases (2031-01-19 -> 2034-12-19)
 - GEOa (2034-12-19 -> 2035-01-17)
 - GCO5000 (2035-01-17 -> 2035-04-16)
 - GEOb (2035-04-16 -> 2035-05-13)
 - TGCO (2035-05-13 -> 2035-05-21)
 - GCO500 (2035-05-21 -> 2035-09-29)
 - All Ganymede phases (2034-12-19 -> 2035-09-29)
 - All mission phases (2031-01-19 -> 2035-09-29)

This method could be generalized to ROIs, CReMA and Maps representations.