readInputTS() : read empty time series

[baptsegu]

By default in ANTARES, input time series files are empty : it means that the load/solar/wind/ror/... is equal to zero.

The current version of readInputTS() returns a warning in that case, for example :
Could not find 'ror' time series.
And the returned data.table does not contain any line for the empty time series.

Instead, it would be more convient to return a data.table which contains one line for every time step and zeros in the load/solar/wind/ror/... column.