Make Model2D a subclass of Model

[hartytp]

In preparation for full support for models with multi-dimensional x-axis data.

Changes:

• Model2D is now a subclass of Model
• WrappedModel now calculates derived parameters
• WrappedModel allows remapping names of derived parameters
• Model2D no longer provides functionality to rename parameters / results - use a WrappedModel for that instead
• Small tweaks to the parameter / result naming in the 2D models for improved consistency
• Fix normalization of FFT in get_spectrum
• Fix FFT-based heuristics in gaussian, lorentzian, sinc and sinc2 models
• Model now has get_num_x_axes and clear_heuristics methods
• Tests can now check accuracy of heuristics as well
• Improve test coverage to start checking heuristics in some models
• WrappedModel rename wrapped_model parameter to model for consistency with other containers
• rename Cone to Cone2D