Beam number format change to :2d

[tjgalvin]

In most instances the filename of output MSs (and images) is of the form:

SB1234.FIELD_NAME.beam1.ms

This is a little annoying when attempting to map a beam number of a sorted list of such names to its position, especially when trying to find items in the prefect UI. We should put beam as a f"{beam_no}:02d" formatted string, and ensure that we are also treating these as strings throughout the code. As far as I recall that is the case, but would need to be sure -- and update the tests around naming.

[tjgalvin]

Addressing this in beamno branch. It seems to have worked by updating the format in the create ms name function within flint.naming.

[tjgalvin]

Closed by #117