Maintain dimension mapping when possible

[axelboc]

This addresses the first part of issue #1572 (cf. first bullet point in #1572 (comment)).

"when possible" means when:

• toggling inspect mode;
• switching to a dataset with no dimension mapper (scalar dataset, group, etc.) and back;
• switching to a visualization with the same number of axes (e.g. between Heatmap and _Matrix _);
• switching to a dataset with the exact same dimensions (both number and size).

If the visualization changes when switching dataset, then the last two conditions must both be met for the dimension mapping state to be maintained.

In other words, the state is maintained when, for instance:

• switching between two NXdata groups with signals with exact same dimensions and interpretations;
• switching between an NXData group with a 1D signal (NX Line), and the signal itself (Line);
• switching between an NXData group with a 2D+ signal with implicit or explicit image interpretation (NX Heatmap), and the signal itself (Heatmap);
• switching between an NXData group with a 2D+ signal with spectrum interpretation (NX Line), and the signal itself if the preferredVis is Line.

However, the state is reset when, for instance:

• switching between two NXData groups with signals with exact same dimensions but different interpretations (e.g. image vs spectrum);
• switching between an NXData group with a 2D+ signal with spectum interpretation (NX Line), and the signal itself (Heatmap) when no preferredVis is set.

[axelboc]

Pretty straightforward in the end 🎉

[loichuder]

Looks good 👏