Active and Passive coefficients of earth pressure to EC7

Calculation of lateral earth pressure coefficients for active, passive and at-rest conditions, for density, surcharge and cohesion. Uses Eurocode 7 Annex C and the UK National Annex. For sloping walls, sloping surfaces, calculates the Ka or Kp coeffients based on peak and constant volume angle of shearing resistance, and coefficient of interface friction for Design Approach 1 Combination 1 and Combination 2. Used in general geotechnical and soil-structure calculations, particularly in cases where material is being retained or where material is resisting actions. Could be modified by changing partial factors for soil materials to suit Design Approach 2 and design Approach 3 in other European locations. Update D4 to correct accidental link between sheets, deal with situations where retained slope is close to angle of internal friction (causing #NUM vaule for DA1/C2 case) and a typo.