fixed issue with gc matching

[avantikalal]

Addresses #156

Earlier in get_gc_matched_intervals, the variable genome was reassigned from the genome name (e.g. hg38) to the genome object produced by genomepy and then passed to filter_blacklist. This led to two issues: 1. filter_blacklist may fail as this is not a blacklist file; and 2. If the user supplied a custom blacklist file, it would be ignored.

Here:

1. The genome object is called genome_obj instead of genome to avoid reassigning the variable name.
2. The user no longer needs to supply the blacklist file separately to get_gc_matched_intervals; the function automatically tries to identify a blacklist file based on the supplied genome. The argument is removed from tutorial 3 as a result.
3. A test for get_gc_matched_intervals is added.