Allow to specify the AnnData var field that has the gene_symbols instead of only relying on var_names about celltypist

[ChuanXu1]

@pcm32, you can provide a symbol-to-ID mapping file (one column being gene symbols and the other column being IDs), and use it to convert the model from gene symbols to Ensembl IDs.

#load a model
model = celltypist.Model.load("some_model.pkl")
#convert the model
model.convert("path_to_your_mapping_file")
#prediction
predictions = celltypist.annotate(input_data, model = model)

In the new version (1.6.1), I also added a mapping file based on GENCODE version 44. So you can use it if you do not find a mapping file yourself. Details can be found in the online tutorial (https://github.com/Teichlab/celltypist -> Usage (classification) -> Supplemental guidance -> Model conversion from gene symbols to Ensembl IDs)