Create for loop wrapper for shift_template_experiment()

[ajlee21]

There is an inconsistency in inputs between the 3 different types of simulation methods (i.e. 2 take gene expression file names while 1 takes gene expression dataframe). This is because shift_template_experiment is run in a for loop so in order to save overhead, we input the dataframe instead.

Its more intuitive to have the user specify a filename so one option is to write a wrapper function that will loop through shift_template_experiment. Raised in #20