DTM refactor and new NRW DTM provider

[kbrandwijk]

Introduces NRWProvider class for handling North Rhine-Westphalia DGM1 data. Integrates data download, merge, reproject, and convert functionalities. Enhances terrain data capabilities with community-driven features.

Refactors DTM provider structure to make base provider responsible for the main flow, and reduce DTM provider implementations' responsibility to downloading tiles.

DTMProvider:

• Implement new processing flow and structure for derived classes
• Added Easy Mode settings to base settings class to apply to all providers
• Added 'base instructions' that are always displayed before provider specific instructions

SRTM:

• Added support for downloading multiple tiles
• Adjusted to use the new derived class implementation pattern

USGS:

• Adjusted to use the new derived class implementation pattern
• Removed all 'custom' logic that wasn't provider specific

NRW:

• Implemented using the new derived class implementation pattern
• Provides DTM 1m data for Germany NRW area