Allow users to bypass deepcopy in from_dict methods

[lossyrob]

As described in stac-utils/pystac-client#49, a lot of time is spent in the deepcopy for a large Item dict when deserializing to a pystac.Item. The deepcopy is done to avoid mutating the parameter dict, which I think is good practice, but also wholly unnecessary when you've just created the dict from e.g. reading from a file or deserializing from a HTTP response and are not planning on using the dict any further. In these cases, the deepcopy is only a performance hit with no benefit.

The goal of this issue is to introduce a parameter to to_dict methods that allow users to avoid copying the dict. This should be used in PySTAC codepaths such as from_file methods where the incoming dict is a temporary data structure.

[lossyrob]

Closed by #454