Move future.pyplot out of future and import it by default

[mantid-builder]

Original Reporter: @FedeMPouzols

The main point here is to do as the title of the ticket says. In principle the module would stay as pyplot. This comes with some additional / side issues that are being described in this small document: https://github.com/mantidproject/documents/blob/master/Design/Plotting/plotting_namespaces_conventions.md

I think/hope there are no issues with this, but as several changes to the user interface are required, please give your thoughts.

Bonus:

• Add a table that says which traditional mantidplot plots are supported in mantidplot.pyplot and which ones are not. There was a list/table in https://github.com/mantidproject/documents/blob/master/Design/Plotting/plotting_cli.md. This could go relatively early in the module documentation, before or after "Plot a Mantid workspace".
  • As for the plotting tool names (plot_spectrum vs. spectrum): support both. List the options available in a table.

[mantid-builder]

This issue was originally trac ticket 11542