Add PySpark Imputation Methods

[dombean]

Add Imputation Methods to rdsa_utils/methods/imputation:

• Rollback Method
• Growth Rate (Forward) Method
• Growth Rate (Backwards) Method

To ensure our rdsa-utils package, particularly within the methods/imputation directory, remains well-organised and user-friendly, we're implementing a clear naming convention for our file structure.

This approach allows us to clearly denote which library (PySpark or Pandas) a file is intended for, using prefixes (pyspark_ or pandas_) followed by the method name, such as rollback, growth_rate_forward, or growth_rate_backward.

This naming strategy facilitates the inclusion of library-specific methods without the need for each method to have a counterpart in the other library, thereby providing flexibility in our development process. It also helps in reducing unnecessary constraints and ensuring that users can easily navigate and identify the functionalities relevant to their needs. For shared logic that applies across both libraries, we use a common_utils.py file.

rdsa-utils/
│
├── methods/
│   ├── imputation/
│   │   ├── pyspark_rollback.py
│   │   ├── pyspark_growth_rate_forward.py
│   │   ├── pyspark_growth_rate_backward.py
│   │   ├── pandas_rollback.py             # Optional, only if exists
│   │   ├── pandas_growth_rate_forward.py  # Optional, only if exists
│   │   ├── pandas_growth_rate_backward.py # Optional, only if exists
│   │   └── common_utils.py                # For shared logic, if any