Rewrite error functions and documentation

[rwst]

This ticket should accomplish:

• implement inverse error function erfinv (Doctest fix for: Sage can't make sense of a Maxima result #11349)
• implement imaginary error function erfi.
• People need to manipulate statistical distribution functions as in https://ask.sagemath.org/question/35407/use-cumulative-distribution-functions/. To provide the means a symbolical version of erfc is necessary. The current error_fcn() is only numerical.
• put it all in a separate file functions/error.py with central documentation

http://dlmf.nist.gov/7

Component: symbolics

Author: Ralf Stephan

Branch/Commit: 1eac4d8

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/21819

[rwst]

Description changed:

--- 
+++ 
@@ -1,2 +1,8 @@
-People need to manipulate statistical distribution functions as in https://ask.sagemath.org/question/35407/use-cumulative-distribution-functions/ 
-To provide the means a symbolical version of `erfc` is necessary. The current `error_fcn()` is only numerical.
+This ticket should accomplish:
+* implement inverse error function `inverf` (#11349)
+* implement imaginary error function `erfi`.
+* People need to manipulate statistical distribution functions as in https://ask.sagemath.org/question/35407/use-cumulative-distribution-functions/. To provide the means a symbolical version of `erfc` is necessary. The current `error_fcn()` is only numerical.
+* make them all `GinacFunction`s
+* put it all in a separate file `functions/error.py` with central documentation
+
+http://dlmf.nist.gov/7

[rwst]

Dependencies: #11349

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Branch: u/rws/rewrite_error_functions_and_documentation

[rwst]

Changed dependencies from #11349 to none

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New commits:

fbb77c0

21819: erf() reorg

[rwst]

Description changed:

--- 
+++ 
@@ -2,7 +2,6 @@
 * implement inverse error function `inverf` (#11349)
 * implement imaginary error function `erfi`.
 * People need to manipulate statistical distribution functions as in https://ask.sagemath.org/question/35407/use-cumulative-distribution-functions/. To provide the means a symbolical version of `erfc` is necessary. The current `error_fcn()` is only numerical.
-* make them all `GinacFunction`s
 * put it all in a separate file `functions/error.py` with central documentation
 
 http://dlmf.nist.gov/7

[rwst]

Author: Ralf Stephan

[rwst]

Commit: fbb77c0

[rwst]

Description changed:

--- 
+++ 
@@ -1,5 +1,5 @@
 This ticket should accomplish:
-* implement inverse error function `inverf` (#11349)
+* implement inverse error function `erfinv` (#11349)
 * implement imaginary error function `erfi`.
 * People need to manipulate statistical distribution functions as in https://ask.sagemath.org/question/35407/use-cumulative-distribution-functions/. To provide the means a symbolical version of `erfc` is necessary. The current `error_fcn()` is only numerical.
 * put it all in a separate file `functions/error.py` with central documentation