Support for flow control

[DrChainsaw]

There is currently very limited (at best) support for controlling the program flow (e.g. compute this x-times or compute this if y, else that, neural ODEs etc.).

A possible approach is to implement some kind of "graph in vertex" concept, where the computation of a vertex may consist of one or more graphs (which may in turn be mutated just as any other graph).

Some of the things needed of the top of my head:

1. Not too cumbersome way to combine graph(s) and control flow so that graphs are accessible for mutation.
   -Maybe a struct with graphs and computation (which uses those graphs in an arbitrary way) as separate fields is enough.
2. Ability to tie sizes of arbitrary vertices together.
   -For example, looping the output back as input obviously requires that output size is same as input size.
   -A use graph1 if x, else graph2 type of vertex also requires that both graph1 and graph2 are aligned with the input/output size of the vertex they are inside.