[FEATURE][QGIS-Server] Add Web Coverage Service support : funded Ifremer

QGIS-Server already supports standards : Web Map Service (WMS version 1.3.0
and 1.1.1) and Web Feature Service (WFS version 1.0.0) and Web Feature
Service with Transaction (WFS-T).
The French Research Institute, Ifremer, would like that QGIS-Server supports
all the OGC standards.
We (3Liz) are implementing the Web Coverage Service (WCS version 1.0.0)
support in QGIS-Server.
Merge pull request #1025 from rldhont/wcs

[FEATURE][QGIS-Server] Add Web Coverage Service support : funded Ifremer