Adding site-specific plugins

[keomabrun]

As each site might need specific functions to perform values conversions before saving data into the database, we need to implement a mechanism that dynamically imports those functions.

We will create a plugins/ directory with each plugin as a .py file.
User will be able to enable the different plugins by filling a list in the config file.
In the Sol Definitions, generic functions names will be given and if the function exists in the active plugins, it will be called.

[keomabrun]

@5ource, I created this issue according to your comment in e0239b4 about the having different functions depending on MAC addresses.

Should we integrate this in the 0.0.5 release ?

[twatteyne]

@keomabrun, is plugins not a bit too generic? I suggest converters or so.

[keomabrun]

@twatteyne, good point. Agreed !