update contact prediction example notebook

[rmrao]

Updates the contact prediction notebook to be compatible with colab and to compute contact precision

• Data is downloaded from github + pdb
• Model weights are downloaded using aria2c
  • This is ugly, but the code seems to hang when downloading on colab using torch hub... curl works without a separate install, but it takes ~5-10 minutes to download the weights. aria is several times faster.
• Code for computing precisions and creating contact map visualizations is added

[rmrao]

Updated, will merge now.