Richvalues

Release of the second minor update of the library. The user guide has been updated with the new features and modifications.

**List of changes**
- Some of the arguments of the functions and classes used to create rich values have been removed. In the case of `num_sf` and `min_exp`, they can be modified changing the same-name attributes of the `RichValue` object. In the case of `allow_log_scale`, it has been removed for the sake of simplicity.
- The main value of the `RichValue` class has been renamed from `center` to `main`. For the `RichArray` class, the method `centers` has been renamed to `mains`. However, a new method called `center` has been added to the `RichValue` class (and `centers` for `RichArray`), that returns the centers of only the rich values that have a main value with uncertainty (centered values). Also, the methods `is_centr` and `are_centr` have been added, they indicate if the corresponding rich value is a centered value.
- The `RichArray` class has been improved. Now, a different domain for every entry is possible, and all of its properties should be accessed through methods. To set some of these properties with a unique value, a new method called `set_params` can be used.
- The criterion for applying approximate uncertainty propagation has been improved. Related with this, some methods have been added to the `RichValue` and `RichArray` classes, mainly `signal_noise` and `prop_factor`.
- Some minor renaming has been applied through the classes and functions of the library.
- Added some minor improvements and bug fixes.

Release of the first minor update of the library. The user guide has been updated with the new features and modifications.

**List of changes**
- Added a function called `errorbar` for easily plotting rich values, which is basically an implementation of Matplotlib's `errorbar` function.
- Added functions for fitting rich values to a model, obtaining model parameters as rich values, called `point_fit` and `curve_fit`; they are based on SciPy's `minimize` function, from the `optimize` submodule.
- Improved function `function_with_rich_values`, which now supports non element-wise operations, with a variable number of outputs.
- Added some methods for the `RichValue` and `RichArray` classes, mainly `interval` and `is_interval` for `RichValue` and equivalent ones for `RichArray`.
- Some attributes for the `RichArray` class were transformed into methods to improve consistency when editing the elements of `RichArray` objects; these modified attributes are mainly `centers`, `uncs`, `are_lolims`, `are_uplims` and `are_ranges`, which are now methods.
- Improved function `evaluate_distr`, which is now called inside `function_with_rich_values`.
- Added shortened names / aliases for the following functions:
- `rich_value`: `rval`.
- `rich_array`: `rarray`.
- `rich_dataframe`: `rich_df`.
- `function_with_rich_values`: `function`.
- `function_with_rich_arrays`: `array_function`.
- Added some minor improvements and bug fixes.

Initial release of the library.

You can check the user guide of this version to see its features and capabilities.