Miscellaneous Improvements

• Adds support for the NeXus definition of data weights defined by the property nxweights. A NXdata function, weighted_data returns the signal divided by the weights.
• If the plot function is called with the keyword argument weights=True, the data are plotted with the weights applied to the signal.
• If the plot function is called with the keyword argument aspect=equal, the axes are scaled by the value of the axis attribute scaling_factor if it is present. For example, this corrects the aspect ratio to be equal in absolute units when the axis values are in lattice units or reciprocal lattice units.
• Adds the NXdata select function to return points from 1D NXdata groups that are divisible by a specified number, optionally combined with an offset value. The default returns the values at all integer axis values (i.e, divisor=1, offset=0).
• Adds extra keyword options to the plot function (e.g., aspect, cmap, interpolation, bad) to enable plots to reproduce NeXpy options more faithfully.
• Allows the 1D smoothing to be set as a multiple of the original axis steps to ensure that the smoothed interpolations include the original points.
• Fixes a bug where a NXdata slab, whose signal is stored in an external file, is returned with the signal name changed to the field name in the external file.