Only build the fancy docstring when in an IPython context

[maximlt]

I was a little surprised to see that Param was always building a docstring for IPython even not in an IPython context. This also came with some side-effects as the pager calls .param.objects() which leads to the Parameters being cached on the class:

param/param/ipython.py

Line 63 in b96169f

params = dict(obj.param.objects('existing'))

This can only improve the performance of loading modules with lots of Parameterized classes that have lots of Parameters, no idea by how much though!

However, this of course also affects how the docstring look like in a non IPython context, e.g. in the Python REPL.

Before (colorized output):

>>> import param
>>> class P(param.Parameterized):
...   """The class docstring"""
...   x = param.Parameter()
... 
>>> p = P()
>>> print(p.__doc__)
params(x=Parameter, name=String)
The class docstring
Parameters of 'P'
=================

Parameters changed from their default values are marked in red.
Soft bound values are marked in cyan.
C/V= Constant/Variable, RO/RW = ReadOnly/ReadWrite, AN=Allow None

NameValue     Type      Mode  

x   None  Parameter  V RW AN 

Parameter docstrings:
=====================

x: < No docstring available >

With this change:

>>> import param
>>> class P(param.Parameterized):
...   """The class docstring"""
...   x = param.Parameter()
... 
>>> p = P()
>>> print(p.__doc__)
params(x=Parameter, name=String)
The class docstring

[jbednar]

Sounds like a good idea!