Rename loom tokenizer function and modify example notebook for adata.

Added anndata tokenizer and switched to Dataset.from_generator

Add instructions to convert gene annotations w/ Ensembl Biomart

Add further explanation regarding input file format for transcriptome tokenizer

Update instructions in tokenizing example to clarify requirement for Ensembl ID

Update instructions in tokenizing example to clarify requirement for Ensembl ID

Add further explanation to tokenizer example script and updated tokenizer to match loompy raised error

Add example for tokenizing .loom dataset