Inconsistent Naming for CSTR Volume Definitions

[schmoelder]

In a recent PR (#245), the definitions of liquid and solid volumes in the CSTR were made more consistent, introducing CONST_SOLID_VOLUME and INIT_LIQUID_VOLUME.

However, with plans to extend the CSTR’s functionality to support pores and multiple particle types, the current naming scheme is no longer sufficient. Specifically:

• INIT_LIQUID_VOLUME should likely be renamed to INIT_BULK_VOLUME to better reflect its role in the context of pores and particles.
• The scope of CONST_SOLID_VOLUME becomes ambiguous: Should it include pore volumes, or should pores be treated separately?

[AntoniaBerger]

In addition, we should delete the deprivation variables INIT_VOLUME and POROSITY.