Rename functions with is_ consistently

[schillic]

This library seems to have the convention that a function name is x is written isx if x is one word, and is_y_z if x is at least two words y z. The exception is if z is type, as in isboundedtype, since this is a modification of isbounded and adding the underscores would obscure that connection.

The renamings done here are:

1. is_polyhedral → ispolyhedral
2. is_hyperplanar → ishyperplanar
3. _is_hyperplane → _ishyperplanar
4. _is_halfspace → _ishalfspace
5. _is_linearcombination → _is_linear_combination

Of these, 1. and 2. are breaking changes. The other three functions are internal. 3. also renames plane -> planar to be closer to the API function.

There are two more API functions starting with is_: is_intersection_empty and is_interior_point. As described above, they fall in the second pattern, so I kept their names. (One can consider renaming is_interior_point closer to in, for instance to in_interior.)