Param

* Fixed error messages for ClassSelector with tuple of classes
* Added get and contains methods for ParamOverrides

A full list of changes since the previous release is available [here](https://github.com/holoviz/param/compare/v1.5.0...v1.5.1).

- Added Range, Color, and Date parameters
- Improved ObjectSelector error messages
- Minor bugfixes

A full list of changes since the previous release is available [here](https://github.com/holoviz/param/compare/v1.4.2...v1.5.0).

- Improved version reporting from version module
- Minor bugfixes

A full list of changes since the previous release is available [here](https://github.com/holoviz/param/compare/v1.4.1...v1.4.2).

* Selector parameters now respect order of options supplied
* Allowed softbounds to be accessed like an attribute

A full list of changes since the previous release is available
[on GitHub](https://github.com/holoviz/param/compare/v1.4.0...v1.4.1).

* Added support for new [ParamNB](https://github.com/ioam/paramnb) project
* Added new parameter types Action, FileSelector, and ListSelector

A full list of changes since the previous release is available
[on GitHub](https://github.com/holoviz/param/compare/v1.3.2...v1.4.0).

* Added Unicode support for param.String.
* Minor bugfixes.

A full list of changes since the previous release is available
[on GitHub](https://github.com/holoviz/param/compare/v1.3.1...v1.3.2).