Merge TemporalData with Data/HeteroData [1/n]

[DamianSzwichtenberg]

This PR attempts to merge TemporalData into Data.
Currently, only edge events are supported. For (src, dst) event definition edge_index is reused as its definition is aligned with our needs. For msg, edge_attr is reused. An additional attribute is time, which represents an event's occurrence time.
New methods include the following:

• is_sorted_by_time(): returns True if data is sorted according to time, False otherwise.
• sort_by_time(): sorts event data according to time.
• snapshot(start_time, end_time): returns event data that occurred in range <start_time, end_time>.
• up_to(time): returns event data that occurred before or at a given time.
• concat(data): returns new data that concatenates the object on which the method was called and given data. Can be used to add new events.

Related issue: #3230

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (747322c) 88.76% compared to head (961eca1) 88.77%.

❗ Current head 961eca1 differs from pull request most recent head 26026ca. Consider uploading reports for the commit 26026ca to get more accurate results

Files	Patch %	Lines
torch_geometric/data/storage.py	87.75%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8454      +/-   ##
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+ Coverage   88.76%   88.77%   +0.01%     
==========================================
  Files         479      479              
  Lines       29955    30034      +79     
==========================================
+ Hits        26589    26664      +75     
- Misses       3366     3370       +4     

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