Create an Index class

[Peter]

daughter of ticket:287

needs ticket:315

Index holds essentially only a SmartPtr?<std::vector<size_t> >.

Index can be created from void, Index, SmartPtr?<std::vector<size_t> >, or from two Index.

In the latter case you will have Index a(b, c) and a(i) equals b(c(i)) i.e. a.size()==c.size()

Further more we have a

size_t operator[](size_t) const;

and a

size_t size(void) const; 

[Peter]

We will probably also need a constructor

Index(size_t n)

that creates a n INdex of length n and for which index[i] = i