Add xeus_zmq_REQUIRED_VERSION cmake

[mcbarton]

We current set the required version in cmake by a variable for other packages but not xeus-zmq. This PR fixes this, and updates the documentation to make clear 1.0.2 is needed as a minimum and not 1.0.0

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.08%. Comparing base (3ef699d) to head (1cf861a).

Additional details and impacted files

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[anutosh491]

I have two things here

1. Let's wait for the update to xeus 5 PR to go in (will do that by today) and then wait for this PR to build on top of that.
2. Also we need to think if a minimum xeus_zmq version is even required. Cause if we're setting a minimum version for xeus .. that by itself might end up being sufficient. For eg xeus 5.0.0 is only comptabile with xeus-zmq 3.0.0 and xeus 4.0.0 (and above) is only comptabile with xeus-zmq 2.0.0 ... so technically ensuring a lower bound of xeus does ensure a lower bound on xeus-zmq

[mcbarton]

I have two things here

1. Let's wait for the update to xeus 5 PR to go in (will do that by today) and then wait for this PR to build on top of that.

2. Also we need to think if a minimum xeus_zmq version is even required. Cause if we're setting a minimum version for xeus .. that by itself might end up being sufficient. For eg xeus 5.0.0 is only comptabile with xeus-zmq 3.0.0 and xeus 4.0.0 (and above) is only comptabile with xeus-zmq 2.0.0 ... so technically ensuring a lower bound of xeus does ensure a lower bound on xeus-zmq

If xeus-zmq minimum version is not required i'll just drop the 1.0.2 from the cmake and remove the xeus-zmq in the readme.

[vgvassilev]

LGTM!