Add API for defining finite element fields by cloning existing fields

[rchristie]

In a lot of places we're defining new fields identically to current fields using the following workaround (between begin/end change):

1. rename field to new name
2. write field of the new name to EX file/buffer
3. restore original field name
4. read file/buffer to define new field

Need direct API to do this efficiently including over subgroup, plus defining fields with different numbers of components based on a chosen component of an existing field.