Added the identifiers for internal copper layers

[mundodisco8]

Quite a silly pull request. I needed to add support to internal copper layers for my personal project, so I thought on generalising for all the internal layers. When I created this pull request I noticed that you had one already that covered internal layers 1 and 2 which cover 4 layer boards, but I still think this pull request is valid as this is a more general case than the other.

[set-soft]

Try "Inner.N", as an example I use:

  - layer: Inner.1
   suffix: GND_Cu
 - layer: Inner.2
   suffix: Power_Cu

Looking at the code N can be any number:

   if s in D:
       layer = PC.LayerInfo(D[s], False)
   elif s.startswith("Inner"):
       m = re.match(r"^Inner\.([0-9]+)$", s)