Add functions to encode/decode identifier ranges of elements or nodes

[rchristie]

For example, between a mesh/nodeset group and either/both:
A list of identifier ranges [start,stop]: [[1,30],[55,55],[66,70]]
A string "1-30,55,66-70".

[rchristie]

Currently adding such functions into scaffoldmaker zinc_utils. Can move across in next push.