added tests to test-complete
further particle tracing speed up. problems:
1) works only for non adaptive problems (but it is probably the case for previous caching as well)
2) hash is created for each particle separately, should be reused. PT will be probably refactored in the future to allow calculations of particle interactions, so all particles will be calculated simultaneously
fixed for adaptivity
- doesn't work for harmonic magnetic field (never worked)
- todo: improve hasForce()