Feature/962 pandas integration

[esloho]

Closes #962

Regarding the second problem exposed in the issue #962 we had to compromised: we'll raise an exception when trying to access with a column Series any method that requires a row Series.

When asking for all the datasets by a particular category (Category.datasets()), that Category instance must represent a single entity like when using loc/iloc in a DataFrame or when using the get_by_id method. But if it represents a column like when accessing a property of the DataFrame (Categories['name']) there's not a single id we can use to search the associated datasets. Ideally these cases shouldn't be represented as Category instances, but since we couldn't find a way to avoid it let's try this solution.

This PR also includes a missing change in last PR: return None (instead of empty list) when no entities are found in the server.

[esloho]

Maybe @oleurud would like to CR this one.

[oleurud]

About the message in the DiscoveryException, I am not sure at all, I think a user is not going to understand the message.

[esloho]

About the message in the DiscoveryException, I am not sure at all, I think a user is not going to understand the message.

I tried different versions and this was the best one, so if you have any suggestion I'd be in debt with you forever and ever ( @alrocar @oleurud guiño guiño)

[oleurud]

Let's go!

A comment for future PRs: I think we can extract some common methods of the entities to a parent class making the code easy to maintain

[esloho]

A comment for future PRs: I think we can extract some common methods of the entities to a parent class making the code easy to maintain

Agree. I tried at first but there wasn't much that could be extracted. I'll give it another look now that we have more methods :) Thanks!

[oleurud]

Acceptance 🍏