* Use logarithm to calculate various quantities.
  In more detail: The values of a0, b0, Xi and Lambda are no longer calculated
  directly, but their logarithms. Some of these values may be very large or
  very small, so the values may overflow doubles.

* New function casimir_get_lmax:
  This function returns lmax.

* Changed summation for Free Energy:
  The summation over n and m can now be done up to a precision of eps. Terms
  that don't contribute will not be calculated.
* Add new function AssertEqual to check if two integers are equal
* add tests for Lambda
* add tests for Xi