[Use case] Adding missing data

[basille]

Use case: GPS data is typically full of missing data, i.e. records with no coordinates because the device could not get satellite signal and gave up. Raw data typically comes with rows including all other information (including time), but empty longitude and latitude. sftraj objects should be able to keep them on creation, or be able to add them if they were not initially present in the data (i.e. add those missing attempts back in the data).

Requirements:

• sftraj objects should be able to include points with no coordinates (NULL); and build steps with these NULL points
• adding missing data should not change the data in any other respect; sftraj objects should be able to remove those NAs and get the initial data back in its exact initial state.

Input:

• If the initial data already has missing data, a data.frame with rows with 0-coordinates.
• If the initial data does not have missing data, a sftraj with gaps in the data.

Output: In both cases, a sftraj object with missing data included.

Additional information: I guess for now, the algorithm to add missing records is beyond the scope of the package (see adehabitatLT::setNA).

[basille]

Closing this issue now that sftrack is developed and on CRAN.