Better core utilisation

Over the past couple of weeks I've modified several parts of the makemap command to split up their calculations between all available threads rather than doing them all in a single thread. This means that makemap should now run significantly faster on multi-core machines. The speed up will be greater for larger data sets than for smaller data sets. As an example, producing a map using a single subarray from a  40 minute observation took  11 mins 49 secs with the new scheme on my 12 core machine, and achieved an average CPU load of 791%. With the old scheme, the same map took 19 minutes 14 secs with an average CPU load of 461 %.