Add option to read in generic ASCII files as a pairwise distance matrix

[alexdi0421]

This allows non-standard, user-defined distance metric in clustering analysis.

[drroe]

This should be possible now as of #915 (may have also been possible before but I'm certain it works now). The ASCII file just needs to have a header with the format:

#F1   F2 <name> [nframes <#>]

Where F1 and F2 are the frame numbers, is the column with the distance, and the optional nframes <#> specifies the total number of frames (in case this is a "sieved" matrix, i.e. it is missing certain frames). This file can be read in with the readdata command and then given to the cluster command via the pairdist keyword.