Move Map and Timeseries and their dependencies to extra requirements

[Cadair]

Given the discussion in #4592 (comment) about adding support for netcdf4 files to Timeseries, I think now is a good (ish) time to reconsider the standing policy that doing pip install sunpy installs all the dependencies for map and timeseries to be fully operational.

The current core deps are:
numpy>=1.16.0
scipy>=1.2.0
matplotlib>=2.2.2
pandas>=0.23.0
astropy>=4.0
parfive[ftp]>=1.1.0

There are a few variants of this plan, but I propose that the best option (other than the status quo) is to add a timeseries and a map extra. This would leave core with numpy, astropy and parfive.

matplotlib is an interesting one, I think we could leave it optional on both map and timeseries, but then cm also wouldn't work, so do we add an extra for that as well? (Maybe we need to leave mpl in core for 2.1 and work these things out??)

Of the rest scipy is Map (and image), pandas is timeseries.

We would need someone to do a good audit of the code make sure we aren't rushing this and miss something though.

[Cadair]

(I think #4596 would alleviate some of my concerns here, by being able to point users at self_test and knowing it will tell them if they are missing deps.)

[dstansby]

I am pro reducing core dependencies, partnered with having clear instructions on how to install all dependencies if desired. As sunpy becomes more established, it seems more likely that there will be use cases of people using it for specific purposes (e.g. just coordinate transforms), and not needing all the dependencies. I think we have known this for a while really, and the concept of sunpy "core" alludes to that.

We would need someone to do a good audit of the code make sure we aren't rushing this and miss something though.

What do you mean by this?

[kakirastern]

Think it just means making sure everything works as expected...

[kakirastern]

Which can be quite a time consuming task if we check everything manually for a genuine audit and not a second identical set of CI/CD checks...

[Cadair]

What do you mean by this?

I just mean people can currently safely make the assumption that we have say scipy available anywhere, say coordinates. Before we can be sure that a dep isn't core we need to work out where they are all being used.

[kakirastern]

Bet my mind is truly literal and would not take a hint 🤣

[wtbarnes]

pandas is also used in sunpy.net.jsoc, both because of the dependency on drms which has a pandas dependency as well as in JSOCClient.search_metadata (which is in deprecated by #4358)