[exceptions] remove unneccessary std::string() around messages

See merge request DennisGlaeser/frackit!22

[test][io][gmsh] use confining domains in test

See merge request DennisGlaeser/frackit!21

Feature/improve geometry sampler

See merge request DennisGlaeser/frackit!18

Feature/entitynetwork use id class

See merge request DennisGlaeser/frackit!20

Moreover, this commit removes the bool in the function arguments to
addSubDomain() to indicate whether or not a sub-domain is to be taken as
confining or not. Now, there are two interfaces:

* addSubDomain(...) -> adds a non-confining sub-domain
* addConfiningSubDomain(...) -> adds a confining sub-domain

Feature/introduce id class

See merge request DennisGlaeser/frackit!17

Feature/first example

See merge request DennisGlaeser/frackit!16

Feature/gmsh writer

See merge request DennisGlaeser/frackit!15

Feature/entity networks

See merge request DennisGlaeser/frackit!14

[pointsampling] fix uniform sampling on cylinder

See merge request DennisGlaeser/frackit!13

Fix/minor typos and docu

See merge request DennisGlaeser/frackit!12