Improve Slocum file retrieval and rename some Slocum file options.

Refactor function to fetch Slocum files with improved options for
date-based filtering, and rename load options with better names.

Also update configuration functions and main scripts.
Improve struct format function with new operations and better names.

Add function call specifier (`@`) and rename existing ones.
Improve JSON functions with faster parsing and some cosmetic fixes.

Avoid the (ab)use of `find` to skip white space.
Fix function to load NetCDF files for the case of no variables.

Introduce a safety check for the inconsiten behavior of MATLAB when
there are no variables to load.

Also set the dataset name (equal to the source name passed).
Change plot order in trajectory and current map plotting function.

Change plot order so water velocity vectors are drawn below
the trajectory, otherwise trajectory might not be visible
when there are to many vectors.
Fix missing handling of date option in figure generation function.

Also change default value to remove dependency on POSIX timestamp function,
and current POSIX timestamp to generated figures in main script.
Improve figure printing options.

Add option to keep intermediate vectorial file,
to enable `-loose` option for uncropped figures,
and to specify the path of the external conversion program.
Always return value in POSIX time mex interface.
Insignificant cosmetic fixes in several functions.
Add FTP-like class for SFTP connections based on libssh.

Class `FTP` provides an interface to operate on files on a remote
server through the FTP protocol. There is no equivalent for SFTP.

Implement the same interface of `FTP` for the SFTP protocol
in a new class `SFTP` using the C library libssh.

Include a helper script to compile the required mex interface to libssh.
Expand retrieval functions to support fetching Seaglider files from basestations.

Refactor common file retrieval code into a lower level function
fetching files from a remote directory using an FTP-like object.

Simplify function for Slocum file retrieval by using the common
low level function.

Add new function to retrieve Seaglider files from basestations.
Add Seaglider data retrieval to real time main script.

Seaglider file and basestation directories are named according
to the serial code (unit) of the glider.
Add the new deployment field `glider_serial` to the deployment info query.

Handle the Seaglider file retrieval in the real time main processing script,
and improve its documentation comment.

Also add basestation configuration function.
Improve mex compilation helper functions.

Do not hardcode paths in the mex compilation helper functions.
Instead, locate dynamically the corresponding .m file
and use its parent directory as compilation root directory.
Add function to report the glider toolbox version.
Report toolbox version in NetCDF product configuration functions and processing logs.
Update delayed time processing outline diagram and Makefile.
Fix some insignificant errors in documentation comments of several functions and whitespace.