corrections and extensions to dynamic slip.

1. Now can be used with exact SGS and LANS-alpha closures.
2. Not using SGS tensor passed in from LES.F90 - instead, calculating it
locally on the face. This is an approximation but is at leats consistent
with the calculation of the test-level SGS tensor. Numerical tests show
a marginal difference in the slip coefficient.
3. improving commenting throughout.
4. Skipping partitions in which no surface elements tagged with
'dynamic_slip' are present.