Read axis values demonstration

[Nathan Mannall (NMannall)]

This PR adds a new XIOS example to demonstrate a bug caused by axis values being overwritten with uninitialised data when reading a file using independent reads.

The demonstration currently fails when running in the CI.

[Nathan Mannall (NMannall)]

This bug is caused by axis values being overwritten with uninitialised data by XIOS when reading a file using independent reads.

The problem function is CNc4DataInput::readFieldVariableValue in file nc4_data_input.hpp: https://forge.ipsl.fr/ioserver/browser/XIOS2/trunk/src/io/nc4_data_input.hpp?rev=2866#L79. Line 84 in that file means that when using independent reads, the function returns and the data in var is never changed (i.e. by reading it from the input file on line 96).

This causes a problem in the function CNc4DataInput::readAxisAttributeValueFromFile in file nc4_data_input.cpp: https://forge.ipsl.fr/ioserver/browser/XIOS2/trunk/src/io/nc4_data_input.cpp?rev=2866#L589

On line 610, the readFieldVariableValue function is called, passing in the uninitialised array readAxisValue. On line 617, this is then used to update axis->value - setting it to uninitialised data.