aggregate_exposures_by_category: Aggregate exposures by category
In eilslabs/YAPSA: Yet Another Package for Signature Analysis

If a valid category (i.e. it matches to a category specified in in_sig_ind_df) is supplied, then the exposures are aggregated over this category.

1	aggregate_exposures_by_category(in_exposures_df, in_sig_ind_df, in_category)

`in_exposures_df`	Input data frame of exposures.
`in_sig_ind_df`	Input data frame of meta information on the signatures. Has to match the signatures in `in_exposures_df`
`in_category`	Category to be aggregated over

A list with entries:

exposures: The exposures H, a numeric data frame with l rows and m columns, l being the number of aggregated signatures and m being the number of samples
norm_exposures: The normalized exposures H, a numeric data frame with l rows and m columns, l being the number of aggregated signatures and m being the number of samples
out_sig_ind_df: Data frame of the type signature_indices_df, i.e. indicating name, function and meta-information of the aggregated signatures..