Unifying date-related arguments into `date`

[sigmafelix]

We have two sets of temporal arguments in amadeus functions (i.e., process_* functions): date or date_start/date_end. How about taking date as character(2) (or Date(2) / POSIXct(2)) in functions which date_start and date_end are used? I will convert these once we will find a consensus.

[mitchellmanware]

That is good with me. I implemented something similar with the process_gmted data where the statistic and resolution are a two character vector (ie. variable = c("Breakline Emphasis", "7.5 arc seconds")).

Passing the two dates into the already created generate_date_sequence() and/or generate_time_sequence() functions, and then taking the first/last observation of each, could reduce the workload to get this done.

[sigmafelix]

Done by #26