Allow callers to stats and multistats API to specify which statistics they are interested in

[corviday]

Currently the stats API returns a JSON object with attributes representing min, max, mean, median, stdev, and ncells values. Plan2adapt's calculations have been running slowly, and it might speed things up a bit if, when plan2adapt requested statistics, it could request only the exact statistics it needs.

Add parameter(s) to control which statistics are calculated.

[corviday]

@rod-glover and I discussed some possible new query designs:

1. A parameter for the presence or absence of each statistical measure:

https://server/api/multistats?ensemble_name=ce_files&model=PCIC12
&variable=cdd&emission=historical,rcp85&time=0&timescale=seasonal
&median=true&stdev=true&min=false&mean=false&ncells=false

2. Or just the parameter alone - any parameter that isn't present in the query won't be calculated:

https://server/api/multistats?ensemble_name=ce_files&model=PCIC12
&variable=cdd&emission=historical,rcp85&time=0&timescale=seasonal
&median&stdev

3. A comma-separated parameter list:

https://server/api/multistats?ensemble_name=ce_files&model=PCIC12
&variable=cdd&emission=historical,rcp85&time=0&timescale=seasonal
&measure=median,stdev

We didn't much like 1, but didn't have strong feelings between 2 and 3. In theory, 3 would be harder to cache, since the parameter list would not necessarily have a consistent order, though in practice a front end will probably ask for the same values in the same order. 2 might require more complicated parameter parsing.

In all cases, not supplying the new parameter would result in receiving all six measures, as is current behaviour.