Coverage of exceptions

[egpbos]

In basic.hpp, there are a few exceptions to safeguard against nullptrs from FFTW plan creation, which for instance can happen if you try to build a plan for a multi-dimensional real FFT with a FFTW_PRESERVE_INPUT flag, because such algorithms are not available in FFTW. These exceptions are there more as a developer safeguard than a user safeguard. Eventually, it would be nicer if such "illegal" combinations of plan parameters were disallowed by the code itself, e.g. by not allowing users to manually specify planner flags, but to hard code all legal possibilities. This will eliminate the need for the exceptions.

Therefore, note to self: don't try to get 100% coverage by making a test with illegal input, or something like that. Just fix the API so that the exceptions can be removed.