Rename processors to match what they do

### Context

The processor names array_express and geo refer to where the data comes from rather than what they do. no_op was originally planned to do nothing, but it turned out we needed to convert gene identifiers.

### Problem or idea

Rename the following:
* array_express -> affymetrix
* geo -> illumina
* no_op -> gene_conversion

### Solution or next step

We'll need to change these everywhere they show up. I think the frontend might use more intuitive names in most places, but we'll need to rename everything in the backend/infrastructure.

Search the frontend, backend, and infrastructure for array_express, geo, and no_op, starting with the processor definitions and their references. The work is done when all uses consistently adopt affymetrix, illumina, and gene_conversion, with no old processor names remaining.

Written by the indexing model from the issue text.