[python][ex2] avoid warning, set mesh size in domain

See merge request tools/frackit!204

[status] fix percentage output to two decimals

See merge request tools/frackit!203

Feature/improve gmsh writer

Closes #17

See merge request tools/frackit!202

Feature/improve entity networks

See merge request tools/frackit!200

[status] suppress warning if no target was set

Closes #22

See merge request tools/frackit!201

One can now set mesh sizes for individual geometries and furthermore,
one can define geometries to be physical or non-physical, depending on
which the interior of the geometries is explicitly meshed or not.

This completely restructures the entity network class, which now allows
for iteration over the fragments of the different constituents
(subdomains, entities, intersections, junctions). Moreover, one can
extract connectivity information on intersections and junctions.

The builder and writer classes are adapted accordingly, and are still
used in the same way as before. The entity network class itself was
modified in a non-backwards-compatible way, as the current
implementation did not provide much functionality apart from passing it
onto a writer, which can still be done in the same way.

The virtual interface now solely defines the basic maps that must be
available, e.g. for writing the network into a brep file.

Instead of storing both a list and a map, we now only store maps of
entity/subdomain fragments -> unique identifiers in order to reduce the
memory overhead. Moreover, the public interface of the entity networks
is changed such that one can no longer get fragments of a sub-domain
with a specific id as the overall map already contains this information.
This can be added in the future as an external function to extract data
of a specific subdomain from the network. The builder classes are also
restructured to no longer store unneccesary data. Only the public
interfaces of entity networks have changed, and due to the small
user-base, and as that interface is currently only used internally by
the writer (which has been modified), we don't guarantee backwards
compatibility here.