added more parameters for more generalized handling:

 * 'tmax' for maximum temperature affecting albedo after Slater et al., 1990
 * 'hmax' set as model fixed parameter (maximum snow height is now 10m)
 * albedo scheme after Denby et al. (need to check ref) now available via namelist (alb_scheme = 'denby')
 * 'mcrit' as critical melting (affects 'isba' and 'denby' albedo scheme)