Move read_table() and Counter() to a top-level utils directory as they are used both by pcigale and pcigale-plots.

Create a pcigale/utils directory to store various utility functions in preparation for the inclusion of cosmology utility functions. Move pcigale/utils.py to pcigale/utils/io.py, which contains only read_table() and adapt the rest of the code accordingly.

When reading a table from a file, convert all the integer columns to float
(except the 'id' column).  These columns are fluxes or error on fluxes and thus
should be treated as float even when they contain only integer.  This solves the
problem of trying to affect NaN value to an integer column.

Fixes #6

When reading a table from a file, convert all the integer columns to float
(except the 'id' column).  These columns are fluxes or error on fluxes and thus
should be treated as float even when they contain only integer.  This solves the
problem of trying to affect NaN value to an integer column.

Fixes #6

When the flux file contains a mix of spaces and tabulations as column separators, pcigale discards the header and takes the first data line as the header. Now pcigale properly handles such a combination. Bug reported by Paola Santini.

Make the source code more compatible with Python 3. As far as I understand the documentation, this should be harmless for Python 2.