set up collection of / extend utility functions

[JuliaSprenger]

Using the neo structure can be quite cumbersome in some cases, eg like slicing objects based on an epoch array (#525). To improve on this we could A) extend the methods neo objects offer and B) start an object-independent collection of utility functions in a separate file. Most likely a combination of both is necessary to achieve best usability of the neo structure.

Here are some suggestions for helpful utility functions:

• [ ] create a neo.Epoch based on a neo.Event object (or maybe two neo.Event objects)
• [ ] implement a time_slice function for neo.Segment that also slices child objects accordingly
• [ ] slicing a neo.Segment based on a neo.Epoch
• [ ] add shift methods for neo Data objects, which move timescale of the object (including t_start and t_stop adjustment, etc)
• [ ] support inplace slicing to avoid data duplication and keep original neo structure.

Any thoughts on this? Additional suggestions?