Make more functions part of the base API

[leouieda]

n_1d_arrays, check_fit_input and least_squares are now part of the
verde.base API. These functions are needed for building new gridders.
Reorganize verde.base into a package and improve the documentation for
least_squares.

Fixes #154

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.