Record compute group costs (Azure)

[timalces]

Aims to resolve #89

• For Azure projects only, records costs for each 'compute group'
• Compute groups are defined by the tag "compute_group" => "groupname"
• Resources must also have the "type" => "compute" tag
• Does not include data out costs
• Costs for each compute group are recorded in their own CostLog with the scope of their 'groupname', for use by the cloud-cost-visualiser. They are not added to the reporter text/slack output
• Also updates how compute costs are identified, namely by using the compute tag, rather than comparing to instance logs