Throw warning when not yet configured

[redfast00]

If you don't follow the instructions and only install the Python library, without installing the other software, you can do the following:

>>> from daqhats import hat_list
>>> hat_list()
[]

If you do follow the instructions and install the other software (https://github.com/mccdaq/daqhats.git and sudo ./install.sh) first, you do get the correct board list.

I think it could be useful to throw an exception if the hal_list function is executed when the daqhats_read_eeproms command has never been executed.