Add default conversion steps for format pairs in convert command

[zhiltsov-max]

Currently, datum convert is a direct shortcut for datum import + datum export, added for better user experience. However, for many input and output format pairs there is a fixed set of transformations and import / export parameters needs to be included to obtain the expected (in most cases) results. That is, it could be helpful for users to reduce the amount of boilerplate work on each conversion.

What is proposed is to add a default conversion operations for format pairs in the convert command, which should be used by default, but can be disabled.