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.

Depends on #374

[abeccati]

Assigning that as initial easy task to familiarize with petascope code.

[Dimitar Misev]

There's no pow equivalent function in rasdaman, I guess that's why it wasn't implemented in petascope in the first place.

So the first task is to implement pow in rasdaman, and then in petascope.

[abeccati]

Opened ticket for rasdaman component (#374), this one will only address petascope component for WCPS compliance.