Change baseline to mean of 1720 to 1800

[safiume]

Per Estimating Changes in Global Temperature since the Preindustrial Period, https://doi.org/10.1175/BAMS-D-16-0007.1
the zero baseline temperature should be the mean temperature for the time period 1720 to 1800.

This issue is to recalibrate the temperature data returned by pymagicc.MAGICCData().

Warnings:

• This issue will not recalibrate radiative forcing data.
• After this issue is completed all MAGICC runs will include the time span of 1720-1800.

Examples:
For example, if the MAGICC starting year is 1700, natively running MAGICC, 1700 would adopt 0ºC. The mean temperature for the years 1720-1800 is then calculated at -0.01738223
When rebased to the mean temperature of the years 1720-1800, the temperature for 1700 is changed to -0.01738ºC.