Feature: High-level network specification (#2050)

Implement a high-level network specification as proposed in
#418. It does not include support for gap junctions to allow the use of
domain decomposition for some distributed network generation.
The general idea is a DSL based on set algebra, which operates on the
set of all possible connections, by selecting based on different
criteria, such as the distance between cells or lists of labels. By
operating on all possible connections, a separate definition of cell
populations becomes unnecessary. An example for selecting all inter-cell
connections with a certain source and destination label is:
`(intersect (inter-cell) (source-label \"detector\") (destination-label
\"syn\"))`

For parameters such as weight and delay, a value can be defined in the
DSL in a similar way with the usual mathematical operations available.
An example would be:
`(max 0.1 (exp (mul -0.5 (distance))))`

The position of each connection site is calculated by resolving th...