Improve loading of event lists for XMM, NuSTAR, Swift, NICER

[matteobachetti]

A number of improvements in how events are loaded, GTI extensions are understoood, and so on.

This was triggered by the Slack request from @bwgref:

Hey all. During Daniela's talk to Shri's X-ray club I think I saw some examples of reading in 
event files with GTIs, etc. But looking through the tutorial notebooks I can't seem to find an 
example of how this is supposed to actually work (so now I'm heading to readthedocs instead). 
Is there a "dealing with real satellite data" notebook example yet? And/or I'm willing to help 
make one if there isn't.

These changes will make the writing of such notebook much easier I hope :)

[dhuppenkothen]

This'll be really useful. However, it needs more documentation and tests. :)

[codecov]

Codecov Report

Merging #502 (e0b2d81) into master (88e2213) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
+ Coverage   95.29%   95.42%   +0.13%     
==========================================
  Files          35       35              
  Lines        5354     5426      +72     
==========================================
+ Hits         5102     5178      +76     
+ Misses        252      248       -4     

Impacted Files	Coverage Δ
stingray/gti.py	98.30% <100.00%> (+0.40%)	⬆️
stingray/io.py	94.03% <100.00%> (+1.34%)	⬆️
stingray/events.py	100.00% <0.00%> (+1.21%)	⬆️

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[dhuppenkothen]

LGTM. Probably means that the documentation needs an update?