Version 0.99.0

Major changes

• Added support to use median when recomputing supercells.
• Introduced n_pc parameter to allow users to specify the number of
  principal components to use in the PCA step when computing supercells.
• Added utils.R file to contain utility functions that are commonly used.

Minor changes

• Cleaned up functions and added improvements to the codebase, in accordance
  with the Bioconductor package development guidelines.
• Updated vignettes to BiocStyle.
• Added more tests.