Oskar unit test

Merge request aims to add a unit test for an OSKAR simulated measurement set. The measurement set can be downloaded from https://www.astron.nl/citt/EveryBeam/OSKAR-single-timeslot.tar.bz2. The unit test in toskar.cc verifies amongst others whether an instance of an OSKAR telescope is returned.

In addition, we aim to check the reproducibility of the Jones matrix at certain pixels, see https://git.astron.nl/RD/EveryBeam/-/blob/oskar-unit-test/cpp/test/toskar.cc#L103-114. However, it turns out that there is a (too) significant difference between results obtained at DAS5 and those returned on the gitlab CI.

Update: issue turned out to be due to the fact that use_channel_frequency_ was left unitialized, thus causing undetermined behavior in the ternary at https://git.astron.nl/RD/EveryBeam/-/blob/oskar-unit-test/cpp/griddedresponse/oskargrid.cc#L111. Simple fix is to set use_channel_frequency_(true) in the constructor initialization list, see https://git.astron.nl/RD/EveryBeam/-/blob/oskar-unit-test/cpp/griddedresponse/oskargrid.cc#L14 (since we never will use the subband_frequency, if I am not mistaken)