Add sequences keyword for make_examples.py make run functions

[RobertRosca]

Would be useful to specify how many sequences you want to create for spb, scs, jungfrau, and fxe runs created by the make run functions. Moving towards using these to create files for some parts of the offline calibration pipeline tests, and as some of the notebooks parallelise across sequence files being able to specify how many to have would help in testing.

Looks like a simple(?) change, I'll try and do this later today unless anybody has comments/suggestions.

[takluyver]

The rough idea so far has been that each function sets up one specific run (like testing with a specified proposal & run number on Maxwell). But I don't see any problem with making the number of sequence files a parameter, so long as the defaults keep the existing behaviour.

I don't think of the mock data machinery as a public API at the moment, though it hasn't changed much since I wrote it a couple of years ago, so it's probably safe enough for us to use in testing other packages.