Check performance of BioAssaySetExporter

[Nicklas Nordborg]

This is slightly related to #1237 and may be caused by the same thing that is causing #903.

[Nicklas Nordborg]

Initial tests seems to indicate that the main factor is the number of selected fields. On the standard 'roles' test data set (4 assays, 35912 spots each) I get:

• ~10 seconds for both serial and matrix formats if ch1+ch2+externalId is selected, the setting for "merge on reporter" doesn't affect the result
• ~70 seconds for MeV format.
• ~70 seconds for both serial and matrix format if ch1+ch2+all reporter fields are selected.
• ~30 seconds if ch1+ch2+about half of the reporter fields are selected.

This is just an indication. More tests with larger data sets are required. There may be other factors as well that doesn't show up until the data sets grow larger.