derived sample_names cannot be modified

[khoroshevskyi]

The sample_name in PEPhub can be derived and modified in the config file.

Unfortunately, because of this, if sample_name does not exist in the sample table, modifying of the project becomes impossible.
An exception occurs when trying to retrieve the sample name from a sample table that was not initialized by peppy. The error is happening in pepdbagent.

Example pep:

pep_version: 2.1.0
sample_table: samples.csv
name: cm4ai_images
description: ''
sample_modifiers:
  append:
    sample_name: key
  derive:
    attributes:
    - sample_name
    sources:
      key: '{Gene_Name}_{Treatment}'

csv file:

AntibodyID	ENSEMBLID	Gene_Name	Treatment	PlateID	Well
CAB005042	ens123	EIF5A	Paclitaxel	3	C2
CAB005042	ens124	EIF5A	Vorinostat	3	C2
CAB005042	ens125	EIF5A	untreated	3	C2