Obsolete parsing and File not found fix

[l-bick]

Fix for #4
The problem is that the obsolete.dat is parsed, but old references are not replaced by newer ones.
I pulled the obsolete last week and still got 'file not found' errors. I could trace it back to the file itself. I added code after parsing, which replaces all cross-references with the latest pdb-ids, replacing all older ones. An example can be shown in the following table:

<	OLD		>	|	<	NEW		>
OBSLTE	13-MAY-20	6L2V	7BYZ	|	OBSLTE	13-MAY-20	6L2V	7DE5
OBSLTE	22-JUL-20	7BYZ	7CH2	|	OBSLTE	22-JUL-20	7BYZ	7DE5
OBSLTE	25-NOV-20	7CH2	7DE5	|	OBSLTE	25-NOV-20	7CH2	7DE5

If you had 6L2V as input, it currently searches for 7BYZ and throws an error and stops.
In the new version, 6L2V gets replaced with 7DE5 and it works flawlessly.

[gahdritz]

Thanks for this! This bug's been around for a while...