Add possibility to group by deployment characteristics

[damianooldoni]

Feature request from @jimcasaer: it's useful sometimes to have effort grouped by deployment characteristics, not only by time periods.

Two alternatives are possible. get_custom_effort() would:

• accept multiple values (vector) for group_by argument: get_custom_effort(mica, group_by = c("day", "locationName")), OR
• add a group_by_feature argument (default value: NULL): get_custom_effort(mica, group_by = "day", group_by_feature = "locationName")

Possible values for these extra grouping are: locationName, tag or a custom column in dp$data$deployment. In other words, all columns characterizing the deployments.

Based on #236, we should opt for first option. In that issue we opted to add an extra value for group_by: deploymentID. This will allow us to merge the two functions, get_custom_effort() and get_effort().

So, this issue is an addition to #236 to allow multiple values for grouping.