Allow negative values for bank level and reference level in Cross section locations tab of Channel layer

[leendertvanwolfswinkel]

This issue was fixed before: #56

But the same issue still exists when you try to set the bank level or reference level to a negative value in the "Cross section locations" tab of the Channel attribute form:

[ldebek]

@leendertvanwolfswinkel Fix available on branch minor_bugs_fixes