Unused attributes and functions in `Materialflow`, `Process`, `Reactor`

[yardasol]

The Materialflow, Process, and Reactor classes contains several attributes and functions that appear to be unused in the code. Specifically, the following attributes and functions are in question:
For Materialflow:

• mass_flowrate
• density
• volume
• temperature
• get_mass()
• __eq__
• __add__

For Process:

• capacity
• volume'
• check_mass_conservation()

For Reactor:

• mass_flowrate
• volume

The purpose and function of each of these attributes should be investigated, and a PR made that either removes or clarifies their purpose.