Do not require credentials for local files

[agrenott]

Avoid adding default srtmv3 data source (and thus requiring earthexplorer credentials) when providing manual input files along with polygon option.

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: -0.04% (target: -1.00%)	100.00%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (8bfcbbf66a2edb38a00e8f3b801ca162b93877e9) | 2336 | 1600 | 68.49% | | | Head commit (2779f1d3ab064a45af695b4956dc0ed46d4d89d4) | 2346 (+10) | 1606 (+6) | 68.46% (**-0.04%**) | **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 (#31) | 18 | 18 | **100.00%** | **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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