TPeak and TGRSIFit

I've added some new fit classes. TGSIFit will be the mother class for all fit classes and inherits from TF1 to make things behave nicely. If you want to fit a peak with TPeak do the following.
- create a new peak with the constructor that takes in the centroid and the peak ranges
```c++
TPeak *myPeak = new TPeak(centroid, range_low, range_high);
```
- you can then fit a histogram using the TPeak as if it is a TF1. However, to get integrals automatically you can run 
```c++
myPeak->Fit(histogram);
```
- The results can be accessed just like a normal TF1 but
```c++
myPeak->Print("+");
```
outputs most of the information one would want.
- I have been thinking about adding the Landau peak that @pcbend has been using with stefan as an option to this class. @pcbend, let me know if this would be something you would want, or we can make a new derived class to do this as well. The nice thing here is the containment of the integrals etc to write to a root file for later.

This class has some potential for bugs, and could use a little upgrading, so please let me know if anyone can think of anything for me to add.