ENH: Geometry: more generic basal_shift

[glyg]

I'm not satisfied with the current handling of this variable:

    @staticmethod
    def update_height(sheet):
        """
        Update the height of the sheet vertices, based on the geometry
        specified in the sheet settings:

        `sheet.settings['geometry']` can be set to

          - `cylindrical`: the vertex height is
             measured with respect to the distance to the the axis
             specified in sheet.settings['height_axis'] (e.g `z`)
          - `flat`: the vertex height is
             measured with respect to the position on the axis
             specified in sheet.settings['height_axis']
          - 'spherical': the vertex height is measured with respect to its
             distance to the coordinate system centers

        In all the cases, this distance is shifted by an amount of
        `sheet.vert_df['basal_shift']`
        """
        w = sheet.settings['height_axis']
        u, v = (c for c in sheet.coords if c != w)
        if sheet.settings['geometry'] == 'cylindrical':
            sheet.vert_df['rho'] = np.hypot(sheet.vert_df[v],
                                            sheet.vert_df[u])
            sheet.vert_df['height'] = (sheet.vert_df['rho'] -
                                       sheet.vert_df['basal_shift'])

        elif sheet.settings['geometry'] == 'flat':
            sheet.vert_df['rho'] = sheet.vert_df[w]
            sheet.vert_df['height'] = (sheet.vert_df[w] -
                                       sheet.vert_df['basal_shift'])

        elif sheet.settings['geometry'] == 'spherical':
            sheet.vert_df['rho'] = np.linalg.norm(sheet.vert_df[sheet.coords],
                                                  axis=1)
            sheet.vert_df['height'] = (sheet.vert_df['rho'] -
                                       sheet.vert_df['basal_shift'])

It seems to me a more generic version could be useful by expanding the (height, basal_shift, height_axis) to a pair of 3D vectors, shifting all the vertices.

This might mean subclassing the geometries

[glyg]

Upon reflection, I am not sure the geometry can be generalized in a meaningful way, as once you added an aribitrary shift vector, you are already close to a "full" 3D cell, in terms of complexity.

[glyg]

Closing in favor of #36