corrected a bug in coords2angles.py

In the previous version of dlpdb (v0.5.0), I introduced a bug into coords2angles.py (and the scripts that depend on it) which has been fixed in this version.

python3 compatility, many bugs fixed, new features

1. I added python3 compatibility for download_pdbs.py and dlpisces.py. (Hopefully these are the last remaining scripts that were incompatible with python3. Warning: I have only tested download_pdbs.py. I have not yet verified that dlpisces.py still works.)
2. I fixed a bug in coords2dihedrals.py
3. I added the ability to calculate "projected dihedrals". (The angle between the infinite lines formed by the first two and last two atoms, respectively.)
4. I fixed numerous packaging bugs that were preventing installation via pip.

There may still be many remaining packaging bugs and python3 compatibility bugs I have not yet discovered. However if the scripts are able to run without crashing, then the calculation of angles and distances should be accurate now.

coords2dihedrals.py now returns angles in the range from -180 to 180 by default

I changed the default branch-of-log for "coords2dihedrals.py" (from 0 to π). This means that now, by default the dihedral angles returned by this script are in the range from [-180,180) instead of [0,360). (This is a much more sensible choice for interpreting dihedral angles in molecules.) This can be overridden by supplying a number (in radians) representing the branch of log (a number from 0 to 2π) specifying where the discontinuity of the angle is located. (https://en.wikipedia.org/wiki/Complex_logarithm).

basic features working

All of the features necessary to automatically download (non-redundant) PDB files and automatically extract distances, angles, and sequence information from them are working. (The DNA coarse grained model featured in the cover image was built using the measurements obtained by this version of the software.)

What's lacking is better documentation and a better interface.
(...and I don't know if I'll ever bother with that, unless someone asks for it. This is not a very popular program.)