WebNetCPU changes.

http/index.html:

*) Comparison drawing/chart of all the tasks whose details are being shown at any given moment.
Such a comparison chart is mainly designed for:
**) determining quality-of-convergence from multiple convergence runs; and
**) heterogeneity-determination in a given dataset: by way of comparing uncorrelated attribute-variances between different models -- those that dont "account for scale" (e.g. mixed-logit) and those that "do" (e.g. gmnl-variants).

*) Rememberance of the chosen column-types in a given csv file when preview-rows values are changed. Old behaviour was to reset the chosen column types whenever the number of priview rows changed, thereby causing frequent re-specification and re-clicking (in some use-cases).

http/server.cc:

*) cache-control, adding 'must-revalidate' directive to expirable/cacheable entities.