Implement `pow()' WCPS function

[Piero Campalani]

Despite all exponentialExpr are implemented in Petascope (see WCPS standard Sec.7.1.17), only pow(C,p) function is missing, and should be implemented despite easily reproducible by means of sequences of products.

Pow is a unary expression (src/main/java/petascope/wcps/server/core/UnaryOperationCoverageExpr.java).

Examples:

for c in (mean_summer_airtemp)
   return encode(
      coverage averagesOverTime
      over     $T t(1:5)
      values   pow($T,3),
   "csv")

for c in (mean_summer_airtemp)
  return encode(
       pow(c[x:"CRS:1"(100:101),y:"CRS:1"(300)],3),
    "csv")

See how other similar operators are implemented, e.g. log, ln, exp.