Enable numerical evaluation of complex valued implemented functions

[riemarc]

Brief description of what is fixed or changed

Consider the the following implemented function f_imp

def f_num(x): return x + 1
f_sym = sp.Function("f", real=False)
f_imp = implemented_function(f_sym, f_num)

At the moment it is only possible to get a numerical value if the return value of f_num is real valued, for example

print(f_imp(1).evalf())

With this pr also

print(f_imp(1j).evalf())

provides the correct complex value.

Release Notes

• core
  • Enable numerical evaluation of complex valued implemented functions

[sympy-bot]

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Here is what the release notes will look like:

• core
  • Enable numerical evaluation of complex valued implemented functions (#20538 by @riemarc)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.8.

Click here to see the pull request description that was parsed.

#### Brief description of what is fixed or changed

Consider the the following implemented function `f_imp`
```
def f_num(x): return x + 1
f_sym = sp.Function("f", real=False)
f_imp = implemented_function(f_sym, f_num)
```

At the moment it is only possible to get a numerical value if the return value of `f_num` is real valued, for example
```
print(f_imp(1).evalf())
```

With this pr also
```
print(f_imp(1j).evalf())
```
provides the correct complex value.


#### Release Notes

<!-- BEGIN RELEASE NOTES -->
* core
  * Enable numerical evaluation of complex valued implemented functions
<!-- END RELEASE NOTES -->

[codecov]

Codecov Report

Merging #20538 (22284aa) into master (ad63fcc) will increase coverage by 0.017%.
The diff coverage is 63.000%.

@@              Coverage Diff              @@
##            master    #20538       +/-   ##
=============================================
+ Coverage   75.740%   75.757%   +0.017%     
=============================================
  Files          673       673               
  Lines       174330    174445      +115     
  Branches     41185     41202       +17     
=============================================
+ Hits        132039    132156      +117     
- Misses       36567     36571        +4     
+ Partials      5724      5718        -6     

[oscarbenjamin]

Closing and reopening to restart the tests.