Add Sonny's LiDAR DTM source

[agrenott]

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

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

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

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (2779f1d)	2346	1606	68.46%
Head commit (14daf44)	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: <coverage of head commit> - <coverage of common ancestor commit>

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: <covered lines added or modified>/<coverable lines added or modified> * 100%

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