[xas_viewer / CSV export] wrong master energy when two groups differs in absorption edge

[maurov]

This is a follow-up of #393

In xas_viewer:

• selects two groups that have two different absorption edges, let's say Fe and Co K-edge
• export groups to CSV
• select master energy scale for Fe group
• the Co spectrum gets saved with Fe energy -> wrong

Solution:

• if a master energy group is selected, check that the others selected groups have same element and edge as the master one.