Improve freeze/thaw usability (#22)

* Add a longer explainer on freeze and thaw

* Add ArrayContext.clone

* with_container_arithmetic: Add _same_cls_check

* Add test for error-on-mixed-array-contexts

* Improvements/fixes to freeze/thaw explainer

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

* Remove trailing whitespace (flake8)

* Add link to lazy eval functionality under the hood of freeze/thaw

* Stop using code injection for actx match checking in with_container_arithmetic

* Make usage guidelines for freeze/thaw a separate section

Co-authored-by: Thomas H. Gibson <gibsonthomas1120@hotmail.com>

* Tweak phrasing in freeze/thaw usage guidelines, add anchors

* Remove an extraneous word in the freeze/thaw description

Co-authored-by: Alex Fikl <alexfikl@gmail.com>
Co-authored-by: Thomas H. Gibson <gibsonthomas1120@hotmail.com>