Fixes for nastruct, ZDNA and parallel DNA

[drroe]

Version 6.18.0.

• ZDNA (left handed) is now properly detected and parameters have the correct sign.
• Improved base pair detection, especially for parallel strands and/or Hoogsteen base pairing. Renders guessbp obsolete.
• Parameters for parallel DNA can now be calculated using 3DNA-type conventions (new default) or Babcock et al. conventions (previous behavior). Can switch with new keyword, bpmode.
• Improved Zp parameter calculation; fixed for parallel DNA.
• Add new data set, <name>[nxyz] for printing the base pair axes normal (Z) vector.

[drroe]

@hainm I added a new data set to nastruct with aspect [nxyz] that is a DataSet_Vector (x y z, ox oy oz). How can I modify nucleic_acid_analysis.py in pytraj so it understands this? Right now the test is failing because everything else is a 1D array.

[drroe]

Since I have people waiting on this functionality I'm going to merge, but I'll open an issue over at pytraj so as not to forget.

[hainm]

but I'll open an issue over at pytraj so as not to forget.

yeah please open a case. Please post the error too.