Return a map of "Frame ranges/ positions" -> "Excel cells/ ranges" on DataFrame.to_excel
e.g.

```python
In[3]: print(f)
Out[3]:
                    Mon                  Tues      Weds       Thur
Meal
Breakfast         Toast                 Bagel    Cereal  Croissant
Lunch              Soup  Something Different!      Rice     Hotpot
Dinner            Curry                  Stew     Pasta    Gnocchi
Midnight Snack  Shmores               Cookies  Biscuits  Chocolate

In[7]: i = f.to_excel("t.xlsx")
In[8]: i
Out[8]:
                         Mon          Tues          Weds          Thur
Meal
Breakfast       <XLCell: B2>  <XLCell: C2>  <XLCell: D2>  <XLCell: E2>
Lunch           <XLCell: B3>  <XLCell: C3>  <XLCell: D3>  <XLCell: E3>
Dinner          <XLCell: B4>  <XLCell: C4>  <XLCell: D4>  <XLCell: E4>
Midnight Snack  <XLCell: B5>  <XLCell: C5>  <XLCell: D5>  <XLCell: E5>

In[10]: i.index.xl
Out[10]: <XLRange: A2:A5>

In[11]: i.columns.xl
Out[11]: <XLRange: B1:E1>

In[9]: i.loc["Lunch", :].xl
Out[9]: <XLRange: B3:E3>

```

The to_excel constructor has all the information needed to create such a map.

This would greatly enhance working with spreadsheet libraries to create graphs etc with exported DataFrames.

For example with xlsxwriter, to create a graph without can look like:

```python
for col_num in range(1, len(calories_per_meal.index) + 1):
    without_chart.add_series({
        'name':       ["Without", col_num, 0],
        'categories': ["Without", 0, 1, 0, 4],
        'values':     ["Without", col_num, 1, col_num, 4]})
```

which to me looks pretty ugly and confusing. Instead with this could become:

```python

for time in calories_per_meal.index:
    xl_linked_chart.add_series({
                        'name': time,
                        'categories': proxy.columns.xl.frange,
                        'values': proxy.loc[time].xl.frange})


```

I actually implemented some subclasses of DataFrame to do this here: 

https://github.com/0Hughman0/xl_link

but I've realised this could become a lot more elegant and less buggy by intercepting the positions of cells further upstream.