READHGT: Import/download NASA SRTM data files (.HGT)

3.0.1.0 with codes included (!)

change USGS web site to mirror for SRTM3 tiles

new option 'login' to specify user and password at NASA/EarthDATA center (free registration needed) in order to download SRTM1 tiles (in addition to 'srtm1' option).

- replaces unzip with webread function, a workaround to automatic download failure

- 'decim' option applies also to output grid(s).

adds new option 'wget' to workaround an issue with older releases (< 2014b), when downloading tiles automatically. This option works only for Linux and MacOSX. See documentation for further explanation.

MAJOR UPDATE: changed URL protocol to access USGS SRTM3 files. This is to avoid "offshore tiles" message when downloading new files. An alternative is to always use the URL option:
readhgt(...,'url','https://dds.cr.usgs.gov/srtm/version2_1')

updates URL for world-wide SRTM1 tiles. Still unofficial...

- fix issue when merging SRTM1 tiles

- fix a problem when SRTM1 tiles are not found.

- fix in AREA syntax

- new options 'outdir' and 'url' (replacing former syntax)
- improves documentation

- new syntax readhgt(AREA)
- now can download SRTM1 30-m resolution tiles worldwide with option 'srtm1'

- 'merge' option is now default. Use new 'tiles' option to plot/download individual tiles when lat/lon are vectors
- LON argument is now automatically normalized to -180/+179°
- dem.m companion function update

improve backward compatibility (strjoin)

corrects an issue with tiles above latitude 54N (thanks to Greg).

- bug correction in case of READHGT(FILENAME)
- rewriting of help
- minor corrections

- new 'interp' option that fills the novalue gaps
- new 'crop' option to select rectangle part of tile(s)
- updated DEM companion function

minor update

- improves coding
- upgrades companion dem.m function

Bug correction when "dem.m" function is not detected.

- minor modifications
- now uses DEM function (included in ZIP file) for basemap axis and legends

- adds possibility to download multiple tiles with merge option
- improved graphic output

- adjust aspect ratio with latitude
- add minimal basemap axis ticks

- now reads also SRTM1 USA territory (1 arc-second resolution) data files.
- minor bug corrections.