as.time.table: Create a time.table
In rossklin/timetablr: Representation and basic operation for multiple time series.

Create a time.table given a data.frame containing its data and information about how to interpret its columns.

1 2	as.time.table(df, id.vars, time.var, measurement.vars = NULL, aux.vars = NULL, frequency = NULL)

`df`	data.frame or data.table containing data, one row for each observation.
`id.vars`	Name(s) of columns identifiying the entity associated to an observation. Each unique combination of values corresponding to one entity.
`time.var`	Single name identifying the column containing time information for each observation.
`measurement.vars`	Name(s) of columns containing measurement values. These are the columns most functions will operate on by default. Will default to all column names not used for indexing/time/auxiliary.
`aux.vars`	Name(s) of columns containing auxiliary measurements. These will be carried along by most functions but will not be mutated. Will defatult to all columns not used for anything else (with measurement.vars having precedence, i.e. if neither `measurement.vars` or `aux.vars` is specified the non-index/time columns will be interpreted as measurements).
`frequency`	list containing values `from`, `to`, and `delta` specifying a (shared) range of the time series and the size of their time steps. Technically optional though some functions require it (in the future there will probably be a more clear separation between regular and irregual time series). time.table is a subclass of data.table (and therefore also of data.frame) and can thus be used like one. Be careful though, as most operations will lose any time.table specific attributes.