Update for MBH98

This update is to give information for two emulations produced of the 
MBH98 reconstruction.  

The first is by Hampus Söderqvist (hasa0038@student.umu.se), who notes 
the following:
I now have a very close emulation of the MBH98 reconstruction. It uses 
the archived proxy lists, with a few exceptions:

For the AD 1400 and AD 1450 steps, the reconstruction is not a linear 
combination of the archived proxies. The correct proxy lists can be 
determined by adding available proxies until the reconstruction is in 
their linear span. It turns out that PCs 3 to 6 of the NOAMER network 
have been replaced with proxies that were not used in these time steps. 
For comparison, the follow-up paper "Long-term variability in the 
El Niño/Southern Oscillation and associated teleconnections" 
lists the first six PCs (table 1, entries 89?94).

There is also an undocumented AD 1650 step with its own set of proxies. 
It is just the AD 1600 set with some additional proxies.
The emulation can be found here: https://github.com/klimatsmart/mbh98emu

The second is by Wahl and Ammann, in 2007, and can be found here:
https://projects.ral.ucar.edu/rc4a/millennium/WA_supplement.html

Daniel J. Brouillette, 25 October 2021