fix OpenMP statement
Median baseline added as another option for computing the baseline.

Using the option median for baseline computatation can be enabled in
Menue->Edit->Cursor Settings->Base

Currently, this is experimental and has the following known limitations:
- selection option "median" is not stored in the stimfit configuration file
- computing the standard deviation (baseSD) is also disabled
when baselineMedian is selected - return InterQuartileRange (IQR) as BaseSD
indicate baseline method in field names of results table