User-defined variables

[forman]

In order to deeper analyse variables of a cube dataset, it is often desired to compute variables from arbitrary band-math expressions.
Users should be able to define new variables and add them to the respective datasets. For example, to analyse the difference of two variables, users should be able to define a new variable chl12_dev = (chl1 - chl2) ** 2.

The user-defined variables should be persisted in the browser's local storage as we currently do not support server-side persistence of user data. To let the server know about user-defined variables, xcube API endpoints that currently receive a variable name in their URL path should then accept an URL-encoded variable assignment expression of the form <var_name>=<var_expr>, where <var_name> is the name user defined variable and <var_expr> is an arbitrary band-math expression.