v2.1

- som_impbatch.m implemets the Imputation SOM 
- som_make.m updates
  - option to use the Imputation SOM 
- som_quality.m updates
  - implements "combined error"
- gtm_make.m and gtm_show.m for rudimentary gtm functionality
- gtm functions from Netlab package in gtm/
  - added gtm_demo1.m with wine dataset
  - added gtmexpimpute.m and gtmmapumpute.m for missing value imputation
  - added gtminit2.m for initialization with given reference vectors
  - added gtmem2.m GTM training with missing data
  - added gtmemseq.m sequential training for GTM
  - added gtmvariance.m helper function for gtmem2.m
  - dist2.m updated to work with missing data
  - 
