some cleanups of the ECL wells code

most importantly, the pressure update by the linear system of
equations is now considered to be mainly a hint for the linear
solver. the peaceman well class thus always calculates the bottom hole
pressure which exactly corresponds to a given state of the grid before
each iteration. (this is because calculating the BHP is really fast
compared to requireing more non-linear iterations.)

also, the well residual is now used for all calculations and has been
simplified a bit.

finally, the wells class now provides more methods which allow to
query its internal state (i.e., surface and reservoir rates, BHP/THP,
rate targets, etc.)
write ECL summary files for wells

so far, the units of the values written to disk are always what
Eclipse calls METRIC and the number of supported quantities is
somewhat limited...