Updating library-to-host output flushing mechanism and codebase maintenance

This release contains a number of fixes to the internal infrastructure. The Autocmake system was updated to its latest version. Some internal functions have been revamped. The Green's functions, boundary integral operators and solvers code have been extensively rewritten to simplify their structure.

BREAKING CHANGE
The output flushing from library to host was managed by a function called host_writer with signature: void host_writer(const char * message, int message_length)
This has been changed with the current version without preserving backwards compatibility
From this version onwards, the output flushing function will have to be passed as a function pointer from the host to the library via the pcmsolver_new function. Moreover, the expected signature for the passed function only needs one argument: typedef void (*HostWriter)(const char * message)

Read the change log for exhaustive information. Consult also the list of issues