`dem_error`: bugfix on the step func

[yunjunz]

Description of proposed changes

Fix the bug that the input step function date in cmd and template file was NOT corrected passed into the design matrix, due to the inconsistent usage of the variable naming, which was introduced in https://github.com/insarlab/MintPy/pull/823 and identified during the recent Ridgecrest example dataset testing (https://github.com/insarlab/MintPy/pull/1180).

• rename stepFuncDate to stepDate in all dem_error related code and docs

• prep_hyp3: add hdf-eos5 metadata extraction

• docs/demo: use list to break lines

Reminders

• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [x] Pass Circle CI and local test (green)
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.