MagmaClustR 1.1.0

Major

• Provide 4 vignettes explaining in detail how the different features of MagmaClustR work on practical examples.
• Implement expand_grid_inputs() to help create customised n-dimensional input
  grids on which to evaluate the GP.
• Implement regularize_data() to project a dataset on a specific input grid,
  (possibly to control the size of the resulting covariance matrices and the associated running time).
• Add an internal 'Reference' column to datasets, to provide an adequate identifier for multidimensional inputs.
• Implement a new version of simu_db() to generate more realistic 2-D datasets.

Minor

• Round inputs to 6 significant digits to avoid numerical errors.
• Generalise the creation of a grid in any dimension when 'grid_inputs' is not
  specified in the prediction functions.