Fix base directory retrieval for read_corpus method.

[VincentDary]

Hello,

I propose the following fix in order to be able to clone the cpu_rec repository and use cpu_rec.py with symbolic link, than copy the corpus and the script. Without this fix, the following problem exist with symbolic link.

$ cd /tmp
$ git clone https://github.com/airbus-seclab/cpu_rec.git
$ ln -s /tmp/cpu_rec/cpu_rec.py /usr/local/bin/cpu_rec.py
$ cd ~
$ cpu_rec.py 
ERROR: No corpus available