[ENH] Accept API keys

[Maarten-vd-Sande]

Is your feature request related to a problem? Please describe.
The current API allows 3 requests per second, but with an api_key this is increased to 10 per second.
https://ncbiinsights.ncbi.nlm.nih.gov/2017/11/02/new-api-keys-for-the-e-utilities/

This would mean a 3 times speedup for the lookup of samples when you add an api key to sraweb.

Describe the solution you'd like
It looks like adding the api key to these lists of arguments would already work: https://github.com/saketkc/pysradb/blob/master/pysradb/sraweb.py#L74-L89

In that case all that needs to be changed is accepting an optional api_key argument to __init__, and changing the sleeps. I would start a PR, but I am not entirely sure which sleep does what, and thus which one needs to be changed.

[saketkc]

Closed via #61