Fix HARK GUI issue

[MridulS]

#199

User will no longer get the import Matplotlib error as I have moved it to inside the function instead of a global level import.

For example this will work without any issue

In [21]: import HARK 

In [22]: from HARK.ConsumptionSaving.ConsIndShockModel import PerfForesightConsumerType

In [23]: PF_params = {
    ...:     'CRRA' : 2.5,           # Relative risk aversion
    ...:     'DiscFac' : 0.96,       # Discount factor
    ...:     'Rfree' : 1.03,         # Risk free interest factor
    ...:     'LivPrb' : [0.98],      # Survival probability
    ...:     'PermGroFac' : [1.01],  # Income growth factor
    ...:     'T_cycle' : 1,
    ...:     'cycles' : 0,
    ...:     'AgentCount' : 10000
    ...: }

In [24]: PFexample = PerfForesightConsumerType(**PF_params) 

In [25]: PFexample.solve()

In [26]: PFexample.solution[0].cFunc
Out[26]: <HARK.interpolation.LinearInterp at 0x7f102555aa50>

In [27]: PFexample.solution[0].hNrm
Out[27]: 50.49994992551661

In [28]: PFexample.unpackcFunc()

In [29]: PFexample.timeFwd()

In [30]: mMin = PFexample.solution[0].mNrmMin

This fix will give an error if the user tries to plot something on the server without having Matplotlib with the proper backend installed.

the plotting wouldn't

In [31]: plotFuncs(PFexample.cFunc[0],mMin,mMin+10)
    ..........
    ..........
ImportError: No module named _tkinter, please install the python-tk package

The plotting one can be fixed by changing the backend to 'agg' (this has to be done before anything Matplotlib related is imported)

In [1]: import matplotlib

In [2]: matplotlib.use('agg')

In [..]: plotFuncs(PFexample.cFunc[0],mMin,mMin+10)

plotFuncs wouldn't throw any error now and it wouldn't do anything as it is implemented to just do plt.show() (which doesn't work for a server without any display) . Should we add the capability to save the figure too with plotting functions by passing an argument like plotFuncs(save_fig=True, show_fig=False)?

[MridulS]

This is good to go, will update other instances of Matplotlib in ConsumptionSaving refactor.