define compression relevance of SAIs

[westram]

Motivation:

Not all SAIs are (always) relevant for selection of shown columns
(in ​compressed alignment view).

Examples:

• Helix numbers:
  • if helix variants are named like "11a", "11b" … the columns containing letters will stay visible, even if no sequence contains data (behavior may be desired or undesired).
  • single digits of a helix number may "disappear" in compressed mode (should not be default)

Solutions:

• define which characters activate columns (when occurring in SAI).
  Possibilities:
  • letters only (=default; status quo)
  • letters and digits
  • anything (but gaps?)
  • nothing (=ignore SAIs)
• use as global setting or define for each SAI

[westram]

[18462] provides a workaround for disappearing helix numbers:

• load an SAI containing base characters where HELIX_NR contains anything
• use SAI calculator to create such an SAI