Consistency check does not detect NaN-values within component DataFrames

[cpschau]

The function n.consistency_check() does not return any information on missing data within the static an dynamic DataFrames of the network components. Although a network remains solvable despite its contained NaN-values, the missing data are likely to be the consequence of a faulty component initialization and might lead to wrong results (e.g. missing efficiency values as observed in PyPSA/pypsa-ariadne#55). Receiving a warning from n.consistency_check() could provide help with debugging models which output such wrong results.