Add option to keep missing data strata for better neighbourhood structures

[AdamCSmithCWS]

Add an option to the prepare_spatial function that would allow the user to retain a minimal set of strata, including those with no data, to create a fully connected neighbours graph.

Will require additional parameters in the models to handle the empty strata.
If all missing data strata can be confidently identified with values > n_strata, then the model changes are relatively minor. Should require just one extra data object to ensure that the dimensions of the spatial parameters are appropriately set (e.g., n_strata_extra).

Will also require an additional dependency in the concaveman package to set the minimal set of retained spatial strata.

Also consider adding a no-data indicator to the prepare_spatial map output, so user can check to see which extra strata have been included.

[AdamCSmithCWS]

see: https://github.com/AdamCSmithCWS/Spatial_Hierarchical_Trend_Models/blob/main/functions/neighbours_define.R
for an example of modification to neighbours function