[docs] explain how to augment a primermap with GC content and length information

[Thomas Gilgenast]

the bias heatmap and spline normalization functions require that the primermap that is passed has keys for the variables that represent the covariate that is being visualized or corrected, respectively

this fact doesn’t appear to be stated anywhere in the documentation

meanwhile, the defaults for these tools reference “GC“ and “length“ as default covariates, but when run with a generic, non-augmented primerfile these defaults will fail

for the cell systems paper, we used this script to augment primermaps https://bitbucket.org/creminslab/comparison-manuscript/src/master/augment_primerfile.py

this script may not be generic enough to incorporate into lib5c (since it relies on an extra file to supply information about the actual sequences of the primers), but some text should be added to the docs to indicate that users must manually add GC and length columns if they wish to use these two tools/functions

marking as trivial for now because these functions are not used that often