Add code to interrupt the execution

[mbakli]

Fixes #232 .

Changes proposed in this pull request:

• Abort the execution in case of an interruption occurs (e.g. the query is being canceled). The interruption check function has been added before every call to the main boost functions such as dijkstra_shortest_paths, dijkstra_shortest_paths_no_init, etc.
• Suppress the -Wconversion warning in src/trsp/trsp.c

@pgRouting/admins

[dkastl]

@cvvergara , shouldn't a PR point to develop branch, or is this one a "special" case?

[cvvergara]

@dkastl This honors cancellation of execution on many functions. Thing that was pending for many years.
So we don't do back porting (aka add to 3.1 then back port to 3.0)

[mbakli]

Changing this pull request to a two commit pull request.