ENH: Allow in- and out-of-sample parameters to be set

[bashtage]

Separate setting of in- and out-of-sample parameters in forecast

[skip ci]

[pep8speaks]

Hello @bashtage! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file arch/univariate/base.py:

Line 109:17: E126 continuation line over-indented for hanging indent
Line 112:17: E126 continuation line over-indented for hanging indent
Line 1195:62: W504 line break after binary operator
Line 1199:65: W504 line break after binary operator
Line 1510:70: W504 line break after binary operator

• In the file arch/univariate/mean.py:

Line 330:60: W504 line break after binary operator
Line 679:47: E126 continuation line over-indented for hanging indent

• In the file arch/univariate/volatility.py:

Line 276:74: W291 trailing whitespace
Line 277:70: W291 trailing whitespace
Line 311:74: W291 trailing whitespace
Line 312:70: W291 trailing whitespace
Line 352:74: W291 trailing whitespace
Line 353:70: W291 trailing whitespace
Line 577:72: W291 trailing whitespace
Line 969:30: E126 continuation line over-indented for hanging indent
Line 1052:100: E501 line too long (112 > 99 characters)
Line 2072:73: W504 line break after binary operator
Line 2735:65: E126 continuation line over-indented for hanging indent