relax SVN archive

Fixes for the N-state model target function setup for when no parameters are optimised.

Fixes for the N-state model introduced by the fixed tensors.

Fixed for the N-state model target functions for when there is nothing to optimisise.

The fixed alignment tensors are no longer included in the parameter vector from _assemble_param_vector().

Added the N-state model _minimise_setup_fixed_tensors() method.

This assembles the fixed tensors into an array for use in the maths_fns code.

The align_tensor.copy() tensor_to arg can now be None.

This is useful for copying between data pipes.

Created the align_tensor.fix() user function for fixing the alignment tensor during optimisation.

The alignment tensor optimisation is now optional within the population N-state model target fn.

Fix for the Grace write_xy_data() function for when data values of None are passed in.

Fix for the get_data() function - the returned data structure is dependent on the errors.

Instead of a list of x, y, error 1, error 2 with the errors set to None if non-existent, only the
required errors are returned.