Deprecate outdated g-function calculation API functions

[MassimoCimmino]

Functions gfunction.uniform_heat_extraction, gfunction.uniform_temperature, gfunction.equal_inlet_temperature and gfunction.mixed_inlet_temperature have been kept in v2.0 despite the introduction of the gFunction class.

v2.1 will introduce a new 'equivalent' solver which can't be implemented in a backwards compatible way due to the use_similarities parameter. Non-uniform discretization can still be implemented (and will be in #158).

These functions do not have any additional functionality relative to the gFunction class.

This issue is to introduce a deprecation warning in these functions pointing out the lack of support for the new solver and encouraging the user to use the gFunction class. It is proposed these functions be removed in the next major release (v3.0).

[j-c-cook]

I think that sounds reasonable to deprecate the older function API's, and I like that you are introducing a deprecation warning. How will pygfunction track upcoming deprecations? Will a v3.0 milestone be created with a note for deprecations?

Edit: Perhaps this issue should be named "Deprecate outdated g-function calculation API functions"?

[MassimoCimmino]

That is a good point. I added a milestone but it seems only issues can be tracked. I will open an issue "Planned deprecations for v3.0" where we can keep a list.