Resolve RAP-196 "Improve defaults"

This MR adjusts a number of default parameters to obtain better self calibration of typical LOFAR datasets. A number of other changes were also made:

• the two slow gain solves are now allowed to have different solution intervals in time. By default, the first slow-gain solve (joint over all stations) is skipped, as it does not appear to be needed for HBA data; however, more experimentation should be done to verify this.
• a way to specify the maximum number of directions to use in each selfcal cycle was added to the strategy. This option is useful to make sure we don't use more directions than needed (e.g., when a given field has a lot of bright sources that would result in many more directions for a given target flux cut than usual).
• the use of a clean mask has been disabled as it was found that WSClean's automasking results in better cleaning of extended sources.
• the reset of operations was improved, as it was found that some old results were sometimes still being used after reset.
• the flux/astrometry check was adjusted to account for multiscale clean (added in an earlier MR).
• the documentation was updated and revised.