[PierreRaybaut]

Closes #30. Part of DataLab-Platform/DataLab#455.

Problem

apply_downsampling only downsampled a curve above 5 times sig_autodownsampling_maxpoints. With the default value (100,000), signals of up to 500,000 points were drawn in full. Its factor, nbpoints // maxpoints, could also draw up to 20 % more points than the maximum (a 599,999-point signal was drawn with 120,000 points).

Changes

• Curves are downsampled as soon as they have more points than sig_autodownsampling_maxpoints.
• The factor is ceil(nbpoints / maxpoints), so that at most maxpoints points are drawn (at most maxpoints + 2 with the min/max downsampling of Min/max curve downsampling preserving peaks PlotPyStack/PlotPy#85).
• The drawn data are updated when the downsampling factor alone changes (before, only a change of use_dsamp updated them).
• Clearer description of sig_autodownsampling_maxpoints: maximum number of points displayed; the data are not modified.
• Release notes (SigimaX 1.1.0) and their French translation. Only the new entry of the doc catalog is added: the other untranslated entries of earlier changes are left out of this PR.

Tests

• New sigimax/tests/adapters_plotpy/test_curve_downsampling.py, covering:
  • the threshold and the factor around maxpoints, 5 × maxpoints and 6 × maxpoints;
  • the option disabled;
  • a change of the factor alone (apply_downsampling);
  • update_item after option changes.
• 6 of these tests fail without the fix.
• Full SigimaX suite: 354 passed locally, with PlotPy develop. The new tests also pass with Min/max curve downsampling preserving peaks PlotPyStack/PlotPy#85 (min/max downsampling).