Respect PYHELICS_INSTALL (#70)

* Respect PYHELICS_INSTALL

Respect the PYHELICS_INSTALL environment variable as described in https://python.helics.org/installation/

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Co-authored-by: Dheepak Krishnamurthy <me@kdheepak.com>

setup.py

@@ -182,7 +182,7 @@ def read(*names, **kwargs):
 CURRENT_DIRECTORY = os.path.dirname(os.path.realpath(__file__))
 
 HELICS_SOURCE = os.path.join(CURRENT_DIRECTORY, "./_source")
-PYHELICS_INSTALL = os.path.join(CURRENT_DIRECTORY, "./helics/install")
+PYHELICS_INSTALL = os.environ.get("PYHELICS_INSTALL", os.path.join(CURRENT_DIRECTORY, "./helics/install"))
 
 DOWNLOAD_URL = "https://github.com/GMLC-TDC/HELICS/releases/download/v{version}/Helics-v{version}-source.tar.gz".format(version=HELICS_VERSION)