document numpy indexing limitations

indexing, two things not possible:
- array[ivar:]
- array[bvars]
because: returns dynamically diff size;
         we don't do that
how you should model it:
    array*bvars or array*[ivar>i for i in ...]

also?
- array[ivarlist] should mean vectorized: [array[ivar0], array[ivar1], ...]
