histogram: Reference file search improved (5)

Quotation marks added to fix string comparison.
histogram: Do not plot 1D histograms twice
histogram: Reference file for 1D histogram fixed

The variable for found files must be evaluated immediately.
downsampling: Use of SPLIT_DROPSUFFIX fixed

A suffix is dropped for filenames of split data while the
filename root of the down sampled data remains the same.
split: Variable SPLIT_DROPSUFFIX removed

It seems to be too confusing which split
file corresponds to which source file.
main: Apply downsampling to dpca data

The principal component analysis is applied to raw data only and
then the projected trajectories are down sampled, not vice versa.