Add an info() function and use in print(IamDataFrame)

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Description of PR

This PR adds an info() function similar to the pandas equivalent (read the docs) and when calling print() on an xarray object, which is the also used in print(df) (via __repr__).

The output in the first-steps tutorial is:

<class 'pyam.core.IamDataFrame'>
Index dimensions:
 * model    : AIM/CGE 2.1, GENeSYS-MOD 1.0, ... WITCH-GLOBIOM 4.4 (8)
 * scenario : 1.0, CD-LINKS_INDCi, CD-LINKS_NPi, ... Faster Transition Scenario (8)
 * region   : R5ASIA, R5LAM, R5MAF, R5OECD90+EU, R5REF, R5ROWO, World (7)
 * variable : ... (6)
 * unit     : EJ/yr, Mt CO2/yr, °C (3)
 * year     : 2010, 2020, 2030, 2040, 2050, 2060, 2070, 2080, ... 2100 (10)
Meta indicators:
   exclude (bool) False (1)

[danielhuppmann]

Rebased and implemented changes suggested by @Rlamboll in danielhuppmann#21:

• adding number of unique items per index dimension and meta indicator type
• smarter use of line width (can be set kwarg so that we can make it nicer in Jupyter notebooks later)
• make showing memory usage optional

[danielhuppmann]

merging as this has no direct implications on any workflow and formatting can be easily adapted later