Master flat no longer correctly included in SOF

[tomasstolker]

The flat correction was not applied by methods such as util_calib and obs_nodding because the UTIL_MASTER_FLAT key is no longer supported in the SOF file by the recent EsoRex recipes. Instead, only the CAL_FLAT_MASTER key can be used.

This issue has been fixed in commit 00cb0f3.

The commit also implements the create_sof parameter in all methods that use a SOF file. When setting to False, a previously created SOF is not overwritten so it can be manually adjusted.