Many operations are lacking sanity checks

[forman]

Expected behavior

A lot of our Python operations are lacking sanity checks and produce confusing stacktraces.

For example, all of our Python operations should raise ValueError with a dedicated message if a mandatory value is missing i.e. None has been passed to a non-nullable parameter.

Actual behavior

Unrelated exceptions are raised because within most operation's implementation code, illegal values, often None values are accessed without checking, or they are passed into lower level API where tey produce exceptions out of our control thereby increasing the length of the stacktrace.

This produces very confusing error messages for API/CLI/GUI users.

Implementation hints:

1. Utilize Like.convert(nullable=True),
2. in package cate.core.op provide function validate_op_input(op_name, input_name, actual_value) which has access to an input's property passed to @op_input

The selected approach should be documented in the developer guide (#77).

Steps to reproduce the problem

Call e.g. pearson_correlation with any of the mandatory inputs set to None.

Specifications

Cate 0.8.0rc2