Add Sonny's LiDAR DTM source

[agrenott]

Initiate new flexible sources framework, preparing redesign of existing HGT sources.

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +5.89% (target: -1.00%)	94.83%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (2779f1d3ab064a45af695b4956dc0ed46d4d89d4) | 2346 | 1606 | 68.46% | | | Head commit (14daf4456367b2cc75f7e9096d71eb50e26b7085) | 2651 (+305) | 1971 (+365) | 74.35% (**+5.89%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#32) | 329 | 312 | **94.83%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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