All imports inside the test cases refer now to paths relative
to TRIQS python library path. For example,
`import vaspio`
is replaced with
`import applications.dft.converters.plovasp.vasio`
In this way all tests can be executed wtih `build_pytriqs` provided
that the package is installed (the same practice as with other
'dft_tools' tests).
Also, the underscore can now be removed from subdirectory names
because there is no more conflict with module names.
