Superpose3d

Updated jacobi_pd to the latest version.
Made corresponding changes to peigencalc.hpp.
The functionality of the library should be unaffected.

I added some comments explaining alternate conventions for representing rotations with quaternions. The code behavior is not effected. (Perhaps it was not necessary to draft a new release over this...)

Removed all for-loops in the code to improve computational efficiency. Thanks to Alfredo Ricci Vásquez for submitting the PR containing these optimizations.

It now avoids crashing when invoked on empty arrays (length 0).

initial stable release. future API change extremely unlikely.

Two big changes: 1) all input arrays (X,x,w) have been generalized. Any C++ object that supports indexing [] can be used, including vector<vector<double>> and double**, for example. 2) BUGFIX: RMSD is now calculated correctly when allow_rescale==true. (Equivalently, when c≠1. Note: The optimal rotation and translation are not effected. They continue to be calculated correctly. However the estimate of the RMSD was incorrect. Now RMSD is reported correctly.) This bug was detected using more rigorous automated testing than I used in the past. Those tests are now included in "tests/test_superpose3d.cpp" and ".travis.yml". Tests for memory safety and code coverage have also been added.