Stop using React's defaultProps

[FilomenoSanchez]

At the moment we are using defaultProps in multiple components. This is deprecated and will be removed in future React versions (see facebook/react#16210) which results in the following warning:

Warning: FV: Support for defaultProps will be removed from function components in a future major release. Use JavaScript default parameters instead

[FilomenoSanchez]

Probably we want to do one of the following two:

export const MoorhenMapCard = (props) => {
    const { initialContour, initialRadius } = { initialContour: 0.8, initialRadius: 13, ...props }

    // ...etc.
}

// And we can now remove this:
MoorhenMapCard.defaultProps = {
    initialContour: 0.8, initialRadius: 13
}

Or instead use:

export const MoorhenMapCard = ({ initialContour = 0.8, initialRadius = 13 }) => {

    // ...etc.
}

Although the second option is only possible when the number of props is small, which is often not the case.