initial commit of dreamlet function

[erflynn]

Initial attempt to set up dreamlet wrapper. Takes as input a pseudobulked count matrix and metadata.
I tried to follow conventions from Dan's examples where possible.
To do items:

• add info about the default processAssays() behavior (which also filters based on min.cells)
• add back in the dreamlet run df
• update function to take ... and/or additional specific params to pass to dreamlet
• add additional documentation (@dtm2451 I'm not sure what you generate the .txt file with?)
• add example usage
• additional testing

[dtm2451]

I make the txt file with document::document(), and the call was: document::document('single_cell/pseudobulk_function.R', output_directory = 'single_cell/', check_package = FALSE).