Add basic metadata validation run after import

[hombit]

Feature request

This proposal is related to #379 and suggests performing checks to validate metadata after the import. These checks should be done by default.

We can do this analysis with row-group stats from _metadata and check:

• _heapix_29 is within [0, 12 * 4**29) range
• Norder, Dir, Npix values match partitions
• dec is within [-90, 90]

Also all these columns should have no nulls

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.