Add the --gaps flag to lalpulsar_MakeSFTDAG.py

Description

This MR adds the --gaps flag to gw_data_find so that it will emit a non-zero return if the frame list that is returned does not cover the requested time. This should hopefully allow condor to catch when there are missing frames and cause the workflow to fail properly. Right now if the cache file produced does not have the requested coverage, the MakeSFTs step is the one the fails, but really it should fail on the datafind step.

I also took the opportunity to improve the code slightly by using f-strings instead of the format() method

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

Once the test passes @ansel-neunzert can have a look at this for review