Use same UV plot naming unit convention as Aspro2

[jean-philippeberger]

In order to provide a uniform way of labelling uv coverage plot axis a it would be nice if OifitsExplorer would follow the Aspro2 convention. This means

1. Replace UCOORD_SPATIAL, VCOORD_SPATIAL by U and V
2. Provide a double axis unit naming (left M\lambda 10^6 rad^-1 and right meters)

[bourgesl]

Remains 2 to be implemented or discarded