Version 2.0: Deprecated index methods

[loicdiridollou]

Describe the bug

Deprecate the following iterms from the code (cf #624)

Index.is_boolean() has been deprecated
Index.is_integer() has been deprecated.
Index.is_floating() has been deprecated.
Index.holds_integer() has been deprecated.
Index.is_numeric() has been deprecated.
Index.is_categorical() has been deprecated.
Index.is_object() has been deprecated.
Index.is_interval() has been deprecated.

Deprecated Grouper.groups()
Deprecated Grouper.grouper()
Deprecated Grouper.obj()
Deprecated Grouper.indexer()
Deprecated Grouper.ax()

Deprecated keyword use_nullable_dtypes in read_parquet()