Fix simData

[ansonrel]

Improvements to the hierarchical clustering;

• more documentation + some details about the 2 options for cluster simulation
• add some checks at the beginning of the function
• .impute_shared_type_genes is now only performed on-non DE genes
• remove 'house keeping genes' simulation by ignoring the first call of .read_branch
• add .impute_type_genes call into .impute_shared_type_genes
• allow .impute_type_genes to accept different 'type' probabilities for each cluster.
• corrected specs definition in .impute_type_genes (clusters assignment and genes were mixed)