relax SVN archive

Renamed the direction cosine function from 'geometry.py' to 'direction_cosine.py'.

Tidy up of the direction cosine function 'maths_fns/geometry.py'.

The references to geometry have all been replaced by direction cosines.

Huge bug fix!

The direction cosine Hessians of the ellipsoid orientational parameters alpha, beta, and gamma were
not being constructed correctly. The second symmetric part of the Hessian of the dy and dz
direction cosines was actually set to the dx value instead of the dy or dz component.

Bug fix.

Removed the z component of the alpha-gamma element of the Dx Hessian. This should be zero!

Tidy up of the direction cosine calculating geometry function.

More modifications to the diffusion tensor weights.

Bug fix.

When the Hessian type is not supplied correctly, self.hessian_type is None and the match statements
cause a failure. This is now handled properly.

A few modifications to the diffusion tensor weights.

A few modifications to the diffusion tensor correlation times.

Updating the manual to reflect the new diffusion tensor and expansion of the abbreviations.

A small bit of inconsequential formatting to the correlation time file was also included.