Adding a link in the documentation, moving libsmet to the plugins subdirectory and small fixes in GRIBIO

SMET now uses fully the new file indexer class and creates indexes at least every 2000 lines (as in SNIO)

Some warnings have been fixed in the tests and all 'string tmp=""' or 'tmp=""' have been replaced by either an empty constructor (that does the same ) or tmp.clear() (that does the same too)

New warnings have been tried to detect variable shadowing (one variable declared with the same name as another one), alignement issues, etc. This produces very verbose output (not all warnings are legitimate) but allowed to fix lots of cases of misalignement and shadowing issues. These options have been left in the CMakeLists but NOT enabled (since these warnings are not so easy to interpret).

The CMakeLists were not making Visual C++ happy. This has been fixed. Several types issues have been fixed. The /Wp64 option of Visual C++ has been turned off, since it reports wrongly some things as errors (dixit Microsoft and my own experience).

An update of libsmet. Added SMETReader::get_filename() and SMETReader::get_header_intvalue(string key) with which a header value can be directly converted into an integer.

Added libsmet.h to MeteoIO.h

A new library for dealing with SMET files has been developed. The plugin SMETIO builds on top of this library now. It provides:
- A SMETReader class, to read SMET files and parse the header info
- A SMETWriter class to write SMET files and set the header info

The SMET library (libsmet.h and libsmet.cc) are completely independent from the rest of MeteoIO. They have their own exception handling and helper functions, thus they can easily be distributed to other parties wishing to read and write SMET files with C/C++.