Reorganise process directories

Previously: all processes are in the same directory, i.e. postprocessor/core/processes/

Why this is a problem:

• Processes that operate on different data types are in the same directory.

Solution: compartmentalise & group the processes as follows:

• postprocessor/core/processes: follow the parameters-process paradigm, run() function of the process takes one pandas.DataFrame as an input.
• postprocessor/core/multisignal: follow the parameters-process paradigm, but the run() function of the process may take more than one pandas.DataFrame as an input. For example, the 'aggregrate' process, which takes a list of DataFrames.
• postprocessor/routines: do not follow the parameters-process paradigm. For example, plotting and data visualisation routines.

Developers can then add processes as needs arise. This directory structure keeps them organised and easy to maintain.