Added spatial visualisation funciton plotSpatial to visualise colData, gene expression, and reduced dimensions.
Added spatial visualisation function plotCovariate to visualise the biolgy, batch, and library size functions estimated by SpaNorm.
Dynamic calculation of df.tps for rectangular tissue sections.
Allow separate specification of df.tps for biology and library size.
Added GLM-PCA approximation through the SpaNormPCA function. The null model is considered to consist of the library size effects, batch effects, and the gene mean.