Allow for addition of optional Attributes to existing Sample Types

[rabuono]

As discussed in the Samples WG, adding optional Attributes to existing Sample Types should be allowed.

Following issue #1199 , the pool of users who can edit a Sample Type will be more clear.

[stuzart]

Introduces a problem for those using an older version of the template, that don't know that new fields have been added, so the older template will no longer be recognised.