fix: link the fits, could not convert string to float.

[ycexiao]

Problem

Follow the instructions in the manual section 3.3.1, page 25.
"In the “Parameters” panel, select the entire “Initial” column. Type “=Fit 1” and then
press “Enter"

Traceback (most recent call last):
  File "/home/yuchen/Documents/diffpy.pdfgui/src/diffpy/pdfgui/gui/errorwrapper.py", line 61, in _f
    return func(*args, **kwargs)
  File "/home/yuchen/Documents/diffpy.pdfgui/src/diffpy/pdfgui/gui/parameterspanel.py", line 253, in applyCellChange
    self.grid_parameters.SetCellValue(row, 0, str(float(value)))
                                                  ~~~~~^^^^^^^
ValueError: could not convert string to float: '=Fit 1'

The error is shown, but the program succeed to link the parameters from "Fit 1".

Proposed solution

Add a condition to distinguish string and float.