Introduced a preprocessor option to use only one of the ionization
fractions, namely the ionized fraction. This reduces the memory footprint
as instead of keeping three full arrays of both the neutral and the
ionized fraction, we only keep three full arrays of the ionized fraction.

To get the old behaviour (both fractions), compile with -DALLFRAC.

This involved moving photonstatistics.f90 to  photonstatistics.F90 since
it now contains preprocessor statements.
Renamed to photonstatistics.F90