Convert specs to more science friendly forms

[coretl]

After consultation with users we would like to do the following:

• Modify existing specs like Spiral to take a spacing rather than a number of points
• Remove the difficult to use Mask and all the regions
• Add in Circleand Polygon as specs that are filled with a grid of points

The spec we drew:

This leads to the following changes:

• Linspace(axis, start, stop, step) created that is almost identical to Line (including the bounded classmethod) but uses np.linspace to generate
• Change spiral to Spiral(x_axis, x_centre, x_diameter, x_step, y_axis, y_centre, y_diameter=None, y_step=None), where self.y_diameter = x_diameter if y_diameter is None else y_diameter and similar for y_step
• Add Circle(x_axis, x_centre, x_diameter, x_step, y_axis, y_centre, y_diameter=None, y_step=None, snake=True), which is filled with a possibly snaking grid
• Add Polygon(x_axis, y_axis, vertices, x_step, y_step=None, snake=True) which is filled with a possibly snaking grid. The maths from https://github.com/DiamondLightSource/scanpointgenerator/blob/master/scanpointgenerator/rois/polygonal_roi.py may be useful
• Delete Mask, and the contents of regions.py, and all code that uses them

Feel free to break this down into smaller sub tickets

Acceptance Criteria

• The above is done and the tests and docs updated