Feature: Read process and process graph files only once

[yardasol]

Background and motivation

SaltProc currently reads the process and process graph files after every depletion step.

Description of idea

We should make changes in the algorithm in the app.py file to make it so that these only need to be read once at the start of runtime.

Implementation details

We'll need to modify the parameters to the reprocess_materials() and refill_materials() functions.

Potential snags

None