Handle fixed position and orientation

[malramsay64]

The alternative wyckoff sites with higher symmetry, require that the position in at least one axis is fixed as well as the orientation.

The simulation should handle fixing these parameters.

[malramsay64]

This requires some changes to the symmetry parser, handling cases with fixed values.

This update is likely closely related to #34 which is a larger update to the parsing of the mathematical expressions

[malramsay64]

There are a number of constraints a site can have on the molecule which is contained within it. These have a rotational component, an x component and a y component. This means that there are 3 parameters which are required to describe the constraints a molecule has to obey to fit into any type of site.

In addition there is the resulting constraints upon the site, being the degrees of freedom which it can move.

These are all properties which should be described as part of the Wyckoff Site, while it probably makes sense to pre-compute the checks of molecular constraints in the creation of the molecule.