first commit after moving local to repo to new place. Implemented own new version of plasmon cross section, also implementing calculation of constant factor gamma

Removed the thread pool in favor of a simple std::vector<std::thread>. We're not creating threads all the time anyway, so there is not really a need for the increased complexity of a pool.

Added first implementation of the field_file option, where a file with additional fields can be specified and will be added to the atomic potentials.

- Moved everything to namespace stemsalabim
- changed file header comment
- improved doxygen comments