Running unit tests in parallel causes all sorts of race condition issues.

Previously the index type was size_t, i.e., also 64 bit, but actually
contained the spectrum number, which was 32 bit, so casting to an int
did not cause trouble. Now we are using the this pointer as index, and
the values exceed 32 bit. This might explain the failing unit tests on
OSX (clang).

This fixes the bug with changing spectrum numbers that would break the
MRU. A simpler solution would have been to simply remove an index from
the MRU when spectrum numbers change. However, for upcoming changes it
will be beneficial to have removed any references to spectrum number
from EventList.